feat(android): in-app updater (backend/Gitea APK), auto-bump versionCode

The app checks {backend}/api/apk/info on launch and, if a newer versionCode
than BuildConfig.VERSION_CODE is offered, shows an update dialog that
downloads {backend}/api/apk into a single wiped-each-time cache file and
hands it to the system installer (FileProvider + REQUEST_INSTALL_PACKAGES).
No APK hoarding; in-place install preserves data (stable signing key needed).

- backend /api/apk/info now returns versionCode (parsed from the release body).
- publish-apk.mjs auto-increments versionCode, sets versionName, and writes
  versionCode/versionName into the Gitea release body.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

android/app/src/main/res/xml/file_paths.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<paths>
+    <!-- The downloaded update APK lives in cacheDir/updates and is shared with the system installer. -->
+    <cache-path name="updates" path="updates/"/>
+</paths>