docs: correct README setup/OOBE + fix data-volume mount

Bring the README and client/README in line with the current product and
release pipeline ahead of making the repo public-facing:

- Highlights: replace the retired "Flutter client in flight" bullet with
  the native-Android-APK-bundled-in-the-image story.
- Quickstart: document the purpose of each compose volume mount, and fix
  the minstrel-data mount (/data -> /app/data) so it matches the image's
  MINSTREL_STORAGE_DATA_DIR and generated artefacts actually persist.
- Configuration: tie MINSTREL_STORAGE_DATA_DIR to the /app/data mount.
- Updating: replace the bogus :v1.0.x scheme with the real tag model
  (:latest, immutable :vYYYY.MM.DD, :main) and note the bundled APK.
- client/README: rewrite the Production section to match release.yml
  (android-release + needs: ordering, non-tag :latest bundle path).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/README.md

@@ -6,17 +6,27 @@ 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
-   Gitea 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/`.
+CI populates this directory in `release.yml` (no Flutter — the client is
+native Android, built by the `android-release` job):
+
+- **Tag release (`v*`):** the `android-release` job builds + signs the
+  APK, uploads it as a workflow artifact, and attaches it to the Gitea
+  release as `minstrel-<TAG>.apk`. The `image-release` job
+  (`needs: android-release`) downloads that artifact into this directory
+  as `minstrel.apk` and writes the computed versionName to
+  `minstrel.apk.version`.
+- **Main push (non-tag `:latest` / `:main`):** no APK is built, but the
+  image still moves `:latest`, so `image-release` pulls the most-recent
+  release's signed APK from the Gitea API and reconstructs its exact
+  versionName — keeping the in-app update channel alive on `:latest`
+  instead of stripping it until the next tag.
+
+Either way, `docker buildx build` bakes both files into the image via
+`COPY client/ /app/client/`.
 
 ## Development
 
-Empty directory works fine — the endpoints return 404 and the Flutter
+Empty directory works fine — the endpoints return 404 and the Android
 update banner stays hidden ("no update channel" graceful degradation).
 
 To smoke-test the update flow locally, drop a real APK + a version