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.

1 files changed, 27 insertions, 0 deletions

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)
+	}
+}