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, 8 insertions, 0 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 {