ci(server): bundle Android APK into image on tag releases (#397 phase 3)

Final phase of the in-app update flow. release.yml on tag pushes
fetches the APK that flutter.yml is attaching to the same release,
drops it into client/ in the build context, and the Dockerfile's
COPY client/ /app/client/ bakes it into the image.

### How it sequences

flutter.yml and release.yml both trigger on tag pushes and run in
parallel on different runners (flutter-ci vs go-ci). flutter.yml
typically finishes APK build + release attachment in 2-5 min.
release.yml polls the release page for up to 15 min for the APK to
appear, then proceeds. If the APK never lands (flutter.yml failure,
network hiccup), release.yml emits a warning and ships the image
without the bundled APK — server returns 404 from /api/client/version,
banner stays hidden, manual download from the release page still
works. Graceful degradation, not a blocker.

### Repo shape

- client/.gitkeep + client/README.md so the directory exists in git
  and the COPY in the Dockerfile always finds something to copy
- .gitignore excludes client/minstrel.apk + .version so accidental
  commits don't bloat the repo
- Dockerfile: COPY --chown=minstrel:minstrel client/ /app/client/

### Why polling vs cross-workflow trigger

Forgejo Actions' workflow_run support varies by runner version; the
polling approach is universally compatible. If/when we standardize on
a runner that handles workflow_run cleanly, the polling step can
become a `needs:` dependency.

Closes #397.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client/README.md

@@ -0,0 +1,35 @@
+# Bundled client assets
+
+Holds the Android APK (`minstrel.apk` + `minstrel.apk.version`) that the
+server serves via `/api/client/version` and `/api/client/apk` for the
+in-app update flow (#397).
+
+## Production
+
+CI populates this directory on tag releases:
+1. `flutter.yml` builds `app-release.apk` and attaches it to the
+   Forgejo release as `minstrel-<TAG>.apk`.
+2. `release.yml` waits for that asset to appear, downloads it into
+   this directory as `minstrel.apk`, writes the tag string to
+   `minstrel.apk.version`, and `docker buildx build` includes both
+   via `COPY client/ /app/client/`.
+
+## Development
+
+Empty directory works fine — the endpoints return 404 and the Flutter
+update banner stays hidden ("no update channel" graceful degradation).
+
+To smoke-test the update flow locally, drop a real APK + a version
+file into this directory:
+
+```sh
+cp /path/to/app-release.apk client/minstrel.apk
+echo "v0.2.0" > client/minstrel.apk.version
+```
+
+## Why a directory and not embed.FS?
+
+The APK is 30-60 MB. Embedding bloats the Go binary and slows
+`go build` for everyone, even when the APK isn't being changed.
+File-on-disk also lets operators override at runtime via volume
+mount on `/app/client/` if they want to ship their own build.