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, 25 insertions, 1 deletions

diff --git a/README.md b/README.md
index ad3d9f5..ee7b507 100644
--- a/README.md
+++ b/README.md
@@ -103,10 +103,16 @@ lectio --format ical --year 2026 --form new           # override the form
 
 - `GET /api/calendar.json?date=…` or `?from=&to=` or `?year=` (+ optional `&form=old|new`)
 - `GET /calendar.ics?year=…` or `?from=&to=` — served as `text/calendar`
+- `GET /source` — plain-text source offer (see [License](#license))
 
 The web endpoints validate every input and cap a request's span at ~5 years
 (1830 days); JSON carries a stable `"schema": "lectio.calendar/1"`.
 
+Both feeds also carry `source` / `license` (JSON) and `X-LECTIO-SOURCE` /
+`X-LECTIO-LICENSE` (iCal), so a subscriber who never opens the HTML UI still
+gets the AGPL source offer. If you fork lectio and run it as a service, set
+`config.SourceURL` to your own repository.
+
 ## `lectio-ui` — the terminal reader
 
 ```sh
@@ -286,4 +292,22 @@ in config for a permanent default.
 
 ## License
 
-MIT — see `LICENSE`.
+Copyright (C) 2026 Łukasz Kasprzak
+
+lectio is free software: you can redistribute it and/or modify it under the
+terms of the GNU Affero General Public License as published by the Free
+Software Foundation, either version 3 of the License, or (at your option) any
+later version.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+PARTICULAR PURPOSE. See the GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License along
+with this program. If not, see <https://www.gnu.org/licenses/>.
+
+Because this is the **Affero** GPL, section 13 applies to `lectio-web`: if you
+run a modified version and let people reach it over a network, you must offer
+those users the source of your modified version.
+
+Full text in `LICENSE`; third-party data attributions in `NOTICE`.