ci: gitea actions for the suites, the image and the APK

Four workflows, none of which existed:

* `tests.yml` — backend, desktop, `:core` and the web app's typecheck+build, in
  parallel. `:core` needs no Android SDK at all, because settings.gradle.kts
  leaves `:app` out when there is none; `:app`'s Robolectric suite therefore
  lives in the APK workflow, where the SDK is installed anyway. Nothing needs a
  running backend — the suites feed on committed fixtures, which is the whole
  reason they were written that way.
* `lint.yml` — ruff, which the repo had been configured for and never run.
* `docker.yml` — builds the image, *starts* it against SQLite and waits for
  /api/health before pushing, then publishes `:latest` and `:<short-sha>` to
  git.timka.su. A tag nobody can run is worse than no tag.
* `android-release.yml` — the release APK on every push to master, signed from
  secrets, with `apksigner verify --print-certs` reporting which key was really
  used and failing if it was the debug one.

Three things the repo needed before any of that could work:

`versionCode` was pinned at 1, so every APK looked like the same version to the
phone and would not install over its predecessor. It now comes from the run
number, with `versionName` from a property, both via one `setting()` helper that
reads a Gradle property or an environment variable.

The release build fell back to the *debug* signing key whenever the keystore was
missing — silently. That is a trap: the APK installs, looks fine, and can never
be upgraded by a properly signed build. `KADENCE_REQUIRE_SIGNING=1` makes that
case fail the build instead, and the release job sets it whenever a keystore
secret is present.

The compose files built an image with no name, so there was nothing to pull.
`image:` now names the registry tag while `up --build` still builds locally.

Also: `android/ci/install-sdk.sh` (idempotent, cacheable, and the one place that
knows `platforms;android-37.0` is spelled with the minor version), the runner as
a compose service with its config and a setup checklist, six make targets, and a
README section. No runner is registered yet — `make ci-runner` is step two of
that checklist.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docker-compose.yml

@@ -24,6 +24,10 @@ services:
       retries: 10
 
   app:
+    # Named as well as built, so the same compose file can run a published image
+    # instead: `KADENCE_IMAGE=git.timka.su/timka123/kadence:latest docker compose pull`.
+    # `up --build` still builds locally and tags it with this name.
+    image: ${KADENCE_IMAGE:-git.timka.su/timka123/kadence:latest}
     build:
       context: .
       dockerfile: Dockerfile