Add a Gitea Actions CI workflow

Ruby syntax check, a require_relative "app" smoke load, and JS syntax
check on every push/PR. No test suite exists yet, so these are cheap
syntax/load checks rather than behavioral tests.

docs/development.adoc

@@ -90,3 +90,11 @@ gpsd -N -n tcp://127.0.0.1:5001
 
 `GET /api/gnss/position` should then report the fake fix (Bratislava
 Castle, in the sentences above) within a couple of seconds.
+
+== Continuous integration
+
+`.gitea/workflows/ci.yml` runs on every push/PR against a self-hosted
+Gitea Actions runner: `ruby -c` on every `.rb` file, a `require_relative
+"app"` smoke check that the app loads without error, and `node --check`
+on every file under `public/js/`. There is no test suite yet — these
+are cheap syntax/load checks, not behavioral tests.