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, 14 insertions, 0 deletions

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;
+}