licence: relicense MIT -> AGPL-3.0-or-later

lectio was MIT, which let anyone take it closed. The concern is not people
selling it -- no licence stops that, and the AGPL does not try to -- but
someone building a proprietary product on it and giving nothing back.

Plain GPL would leave the obvious hole open: lectio-web is a network
service, and hosting is not distribution, so a modified lectio-web could be
run as a paid subscription API without ever publishing a line. AGPL section
13 closes exactly that.

LICENSE is the verbatim FSF text. README carries the standard notice.

Section 13 requires a modified network-reachable version to PROMINENTLY
offer its source to the users interacting with it, so the offer ships with
the code rather than living only in a file nobody fetches:

  - GET /source          plain text, no template or config dependency, so
                         it answers even when something else is broken
  - page footers         every full page (fragments render inside one)
  - JSON envelope        "source" / "license"
  - iCal header          X-LECTIO-SOURCE / X-LECTIO-LICENSE

The feed fields are not redundant: an /api/calendar.json consumer or an
.ics subscriber never loads a page, so the footer alone would miss them.

config.SourceURL is the single source of truth, and says in its comment
that a fork running as a service must repoint it -- an offer that leads to
someone else's code is not an offer.

Tests pin all of it. This is a licence obligation, not a feature, so it
should fail loudly if a later change drops it.

12 files changed, 186 insertions, 5 deletions

diff --git a/internal/calfeed/ical.go b/internal/calfeed/ical.go
index bce3710..810847d 100644
--- a/internal/calfeed/ical.go
+++ b/internal/calfeed/ical.go
@@ -4,6 +4,8 @@ import (
 	"strconv"
 	"strings"
 	"time"
+
+	"github.com/lukaszkasprzak/lectio/internal/config"
 )
 
 // icalEscape neutralises RFC-5545 TEXT specials AND all CR/LF, so untrusted
@@ -58,6 +60,12 @@ func ICal(form string, days []DayView, stamp time.Time) []byte {
 	add("PRODID:-//lectio//calendar//EN")
 	add("CALSCALE:GREGORIAN")
 	add("METHOD:PUBLISH")
+	// AGPL-3.0 §13 offer for subscribers who only ever see the .ics feed.
+	// X- properties are the RFC-5545 extension point; unknown ones are
+	// ignored by clients, so this is inert for consumers and present for
+	// anyone who looks.
+	add("X-LECTIO-SOURCE:" + icalEscape(config.SourceURL))
+	add("X-LECTIO-LICENSE:" + icalEscape(config.License))
 	add("X-WR-CALNAME:" + icalEscape(calName(form)))
 	ds := stamp.UTC().Format("20060102T150405Z")
 	for _, d := range days {
diff --git a/internal/calfeed/ical_test.go b/internal/calfeed/ical_test.go
index fb7ee9a..215c0e3 100644
--- a/internal/calfeed/ical_test.go
+++ b/internal/calfeed/ical_test.go
@@ -4,6 +4,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/lukaszkasprzak/lectio/internal/config"
 )
 
 func TestICalEscapeInjection(t *testing.T) {
@@ -19,6 +21,22 @@ func TestICalEscapeInjection(t *testing.T) {
 	}
 }
 
+// TestICalCarriesSourceOffer pins the AGPL-3.0 §13 offer into the VCALENDAR
+// header. Someone who subscribes to /calendar.ics in their calendar app sees
+// no lectio page at all; the feed is their entire view of the program, so it
+// carries the offer. X- properties are inert to clients that ignore them.
+func TestICalCarriesSourceOffer(t *testing.T) {
+	out := string(ICal("new", nil, time.Date(2026, 7, 27, 12, 0, 0, 0, time.UTC)))
+	for _, want := range []string{
+		"X-LECTIO-SOURCE:" + icalEscape(config.SourceURL),
+		"X-LECTIO-LICENSE:" + icalEscape(config.License),
+	} {
+		if !strings.Contains(out, want) {
+			t.Errorf("missing %q in:\n%s", want, out)
+		}
+	}
+}
+
 func TestICalStructure(t *testing.T) {
 	days := []DayView{{
 		Date: "2026-01-06", Season: "time-after-epiphany", Week: 1, Colour: "white",
diff --git a/internal/calfeed/json.go b/internal/calfeed/json.go
index c1a50ca..1cb0b6f 100644
--- a/internal/calfeed/json.go
+++ b/internal/calfeed/json.go
@@ -1,17 +1,28 @@
 package calfeed
 
-import "encoding/json"
+import (
+	"encoding/json"
+
+	"github.com/lukaszkasprzak/lectio/internal/config"
+)
 
 const Schema = "lectio.calendar/1"
 
 // JSON renders days as the stable lectio.calendar/1 envelope.
+//
+// "source" and "license" carry the AGPL-3.0 §13 offer to consumers who only
+// ever see this endpoint and never load the HTML UI. They are envelope
+// metadata, not day data -- adding them does not change the schema version,
+// since existing consumers read "days".
 func JSON(form string, days []DayView) ([]byte, error) {
 	if days == nil {
 		days = []DayView{}
 	}
 	return json.MarshalIndent(struct {
-		Schema string    `json:"schema"`
-		Form   string    `json:"form"`
-		Days   []DayView `json:"days"`
-	}{Schema, form, days}, "", "  ")
+		Schema  string    `json:"schema"`
+		Form    string    `json:"form"`
+		Source  string    `json:"source"`
+		License string    `json:"license"`
+		Days    []DayView `json:"days"`
+	}{Schema, form, config.SourceURL, config.License, days}, "", "  ")
 }
diff --git a/internal/calfeed/json_test.go b/internal/calfeed/json_test.go
index d2bc856..aec3d14 100644
--- a/internal/calfeed/json_test.go
+++ b/internal/calfeed/json_test.go
@@ -3,6 +3,8 @@ package calfeed
 import (
 	"encoding/json"
 	"testing"
+
+	"github.com/lukaszkasprzak/lectio/internal/config"
 )
 
 func TestJSONShape(t *testing.T) {
@@ -33,3 +35,28 @@ func TestJSONShape(t *testing.T) {
 		t.Fatalf("bad day: %s", b)
 	}
 }
+
+// TestJSONCarriesSourceOffer pins the AGPL-3.0 §13 offer into the envelope.
+// A consumer of /api/calendar.json may never load a single HTML page, so the
+// footer link does not reach them -- the feed itself has to say where the
+// source is. Dropping these fields is a licence-compliance regression, not a
+// cosmetic one, which is why it is asserted rather than left to review.
+func TestJSONCarriesSourceOffer(t *testing.T) {
+	b, err := JSON("new", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var out struct {
+		Source  string `json:"source"`
+		License string `json:"license"`
+	}
+	if err := json.Unmarshal(b, &out); err != nil {
+		t.Fatal(err)
+	}
+	if out.Source != config.SourceURL {
+		t.Errorf("source = %q, want %q", out.Source, config.SourceURL)
+	}
+	if out.License != config.License {
+		t.Errorf("license = %q, want %q", out.License, config.License)
+	}
+}
diff --git a/internal/config/config.go b/internal/config/config.go
index 31e9078..b6c9e0e 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -64,6 +64,23 @@ const configHeader = `# lectio configuration (INI). Full-line comments only (# o
 // -v/--version output (lectio, lectio-ui, lectio-web).
 const Version = "0.44.0"
 
+// SourceURL is where lectio's corresponding source can be obtained. It is
+// not decoration: lectio is AGPL-3.0-or-later, and section 13 requires a
+// modified version reachable over a network to "prominently offer" its
+// source to the users interacting with it. lectio-web therefore surfaces
+// this on every page (footer -> GET /source) and inside both machine feeds
+// (JSON envelope, iCal X-LECTIO-SOURCE), so an API consumer who never sees
+// the HTML still gets the offer.
+//
+// If you fork lectio and run it as a service, point this at YOUR source.
+// Leaving it aimed here while serving modified code does not satisfy §13 --
+// the offer must lead to the version actually running.
+const SourceURL = "https://github.com/lukaszkasprzak/lectio"
+
+// License is the SPDX identifier lectio ships under, paired with SourceURL
+// wherever the §13 offer is made.
+const License = "AGPL-3.0-or-later"
+
 // validVersions are the scripture versions lectio understands. All are
 // embedded corpora; the former "bt" (the niedziela.pl modern scrape) is gone
 // and a legacy config carrying it is migrated to "wuj" on load (see Load).
diff --git a/internal/web/server.go b/internal/web/server.go
index 6e76d3f..92cb758 100644
--- a/internal/web/server.go
+++ b/internal/web/server.go
@@ -74,6 +74,7 @@ func NewServer(cfg config.Config) http.Handler {
 	mux.HandleFunc("GET /calendar.ics", func(w http.ResponseWriter, r *http.Request) { calendarICSHandler(s.get())(w, r) })
 	mux.HandleFunc("GET /reader", func(w http.ResponseWriter, r *http.Request) { readerHandler(s.get(), s.table())(w, r) })
 	mux.HandleFunc("GET /theme.css", func(w http.ResponseWriter, r *http.Request) { themeCSSHandler(s.get())(w, r) })
+	mux.HandleFunc("GET /source", sourceHandler)
 	mux.HandleFunc("GET /settings", settingsGet(s))
 	mux.HandleFunc("POST /settings", settingsPost(s))
 	mux.HandleFunc("POST /reader/bookmark", addBookmark(s))
@@ -691,6 +692,32 @@ func themeCSSHandler(cfg config.Config) http.HandlerFunc {
 	}
 }
 
+// sourceHandler serves the AGPL-3.0 §13 source offer: where to obtain the
+// code this server is running. Every page footer links here, so a user
+// interacting with lectio-web over a network is never more than one click
+// from the corresponding source.
+//
+// Deliberately dependency-free (no template, no config read): it must answer
+// even when config is broken or a template failed to parse, because a §13
+// offer that only works on healthy days is not an offer. Plain text keeps it
+// readable by both a browser and curl.
+func sourceHandler(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+	w.Header().Set("X-Content-Type-Options", "nosniff")
+	fmt.Fprintf(w, `lectio %s
+Licence: %s (GNU Affero General Public License, version 3 or later)
+Source:  %s
+
+lectio is free software. You may use, study, share and modify it under the
+terms of the AGPL. The full licence text ships with the source as LICENSE.
+
+Because this is the Affero GPL, section 13 applies to this server: if the
+code running here has been MODIFIED, whoever operates it must offer you the
+modified source. If this address does not lead to the version you are
+talking to, that is the operator's obligation to fix, not lectio's.
+`, config.Version, config.License, config.SourceURL)
+}
+
 // settingsData drives templates/settings.html.
 type settingsData struct {
 	Cfg            config.Config
diff --git a/internal/web/server_test.go b/internal/web/server_test.go
index 57286d5..42de0b5 100644
--- a/internal/web/server_test.go
+++ b/internal/web/server_test.go
@@ -630,3 +630,52 @@ func TestCalendarPDF(t *testing.T) {
 		t.Errorf("not a pdf (len=%d)", len(b))
 	}
 }
+
+// TestSourceOffer covers the AGPL-3.0 §13 obligation: a user interacting with
+// lectio-web over a network must be offered the corresponding source. That is
+// a licence term, not a feature, so it is pinned here -- if someone later
+// deletes the footer or the route, this fails rather than quietly putting the
+// operator out of compliance.
+func TestSourceOffer(t *testing.T) {
+	cfg := config.Default()
+	cfg.Offline = true // no network; the offer must not depend on readings
+	srv := NewServer(cfg)
+
+	t.Run("/source names the licence and where to get the code", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		srv.ServeHTTP(rec, httptest.NewRequest("GET", "/source", nil))
+		if rec.Code != http.StatusOK {
+			t.Fatalf("status = %d, want 200", rec.Code)
+		}
+		if ct := rec.Header().Get("Content-Type"); !strings.HasPrefix(ct, "text/plain") {
+			t.Errorf("content-type = %q, want text/plain", ct)
+		}
+		body := rec.Body.String()
+		for _, want := range []string{config.SourceURL, config.License, config.Version} {
+			if !strings.Contains(body, want) {
+				t.Errorf("body missing %q:\n%s", want, body)
+			}
+		}
+	})
+
+	// Every page a user can actually land on carries the link. Fragments
+	// (htmx partials) are excluded -- they render inside a full page that
+	// already has it.
+	for _, path := range []string{
+		"/?date=2026-07-22&v=wuj",
+		"/reader?v=wuj&book=John&chapter=1",
+		"/settings",
+		"/bookmarks",
+	} {
+		t.Run("footer on "+path, func(t *testing.T) {
+			rec := httptest.NewRecorder()
+			srv.ServeHTTP(rec, httptest.NewRequest("GET", path, nil))
+			if rec.Code != http.StatusOK {
+				t.Fatalf("status = %d, want 200", rec.Code)
+			}
+			if !strings.Contains(rec.Body.String(), `href="/source"`) {
+				t.Errorf("page has no source link (AGPL §13):\n%s", rec.Body.String())
+			}
+		})
+	}
+}
diff --git a/internal/web/static/base.css b/internal/web/static/base.css
index d9d0e59..d367c15 100644
--- a/internal/web/static/base.css
+++ b/internal/web/static/base.css
@@ -296,3 +296,17 @@ a {
 /* Export links (download the readings). */
 .export { font-family: var(--font-ui); font-size: 0.85rem; }
 .export a { margin: 0 0.15rem; }
+
+/* Licence footer: the AGPL-3.0 §13 source offer, on every full page.
+ * Quiet by design but never hidden -- §13 asks for a PROMINENT offer, so
+ * this must not be display:none'd or themed to invisibility. Colour (border
+ * and text) is the theme's, per this file's no-colour rule. */
+.licence {
+  max-width: var(--measure);
+  margin: var(--space-4) auto 0;
+  padding-top: var(--space-2);
+  border-top-width: 1px;
+  border-top-style: solid;
+  font-family: var(--font-ui);
+  font-size: 0.75rem;
+}
diff --git a/internal/web/templates/bookmarks.html b/internal/web/templates/bookmarks.html
index efc988a..770c73a 100644
--- a/internal/web/templates/bookmarks.html
+++ b/internal/web/templates/bookmarks.html
@@ -46,5 +46,7 @@
   {{end}}
 
 </div>
+
+<footer class="licence"><a href="/source">source</a> · AGPL-3.0-or-later</footer>
 </body>
 </html>
diff --git a/internal/web/templates/index.html b/internal/web/templates/index.html
index 762ed7b..b99725d 100644
--- a/internal/web/templates/index.html
+++ b/internal/web/templates/index.html
@@ -90,5 +90,9 @@
   <div id="pane">{{.Reading}}</div>
 
 </div>
+
+{{/* AGPL-3.0 §13: the source offer must be visible to anyone using this
+     server over a network, so it sits on every full page, not just here. */}}
+<footer class="licence"><a href="/source">source</a> · AGPL-3.0-or-later</footer>
 </body>
 </html>
diff --git a/internal/web/templates/reader.html b/internal/web/templates/reader.html
index ef144cc..36cc8a1 100644
--- a/internal/web/templates/reader.html
+++ b/internal/web/templates/reader.html
@@ -83,5 +83,7 @@
          onchange="document.body.classList.toggle('mono', this.checked)"> {{.L.Mono}}</label>
 
 </div>
+
+<footer class="licence"><a href="/source">source</a> · AGPL-3.0-or-later</footer>
 </body>
 </html>
diff --git a/internal/web/templates/settings.html b/internal/web/templates/settings.html
index f4fae47..3ae2f41 100644
--- a/internal/web/templates/settings.html
+++ b/internal/web/templates/settings.html
@@ -104,5 +104,7 @@
   </form>
 
 </div>
+
+<footer class="licence"><a href="/source">source</a> · AGPL-3.0-or-later</footer>
 </body>
 </html>