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.

4 files changed, 69 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)
+	}
+}