ci: adopt the modern calendula pipeline + Codeberg mirror

Port Calendula's current CI/release pipeline:

- ci.yaml: pull_request-triggered, change-scope classification
  (docs/metadata-only PRs skip the Android build but still report a
  green CI), and a reproducible-release invariant guard.
- release.yaml: the committed versionName is the source of truth — a
  bump reaching main triggers the release, which builds, signs,
  publishes to the F-Droid repo, then mints the vX.Y.Z tag + Gitea
  release and mirrors it to Codeberg with the signed APK + SHA-256
  checksum. workflow_dispatch runs the re-sign-only recovery path.
- Gitea releases are flagged as pre-releases while MAJOR is 0.
- build.gradle.kts: reproducible-release invariants (vcsInfo,
  dependenciesInfo) + a releaseTest variant for the on-device gate.
- fastlane/ becomes the single source of truth for store metadata;
  the localized F-Droid layout is generated from it at release time.
- Port scripts/, .gitea/ISSUE_TEMPLATE/, and rewrite docs/RELEASING.md
  for the versionName-in-main model; fix stale references elsewhere.

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

docs/ARCHITECTURE.md

@@ -243,7 +243,7 @@ the `@IoDispatcher`). `AgendulaApp` is the `@HiltAndroidApp` entry point;
 | UI | Compose BOM 2026.05.01, Material3 `1.5.0-alpha21` (Expressive APIs), Glance 1.1.1 (widget, later) |
 | Other | DataStore, kotlinx-datetime, kotlinx-coroutines |
 | Tests | JUnit5 (Jupiter) + Truth + Turbine + coroutines-test; the data source is the JVM-testable seam |
-| Versioning | git tag is the source of truth; `versionCode = MAJOR*10000 + MINOR*100 + PATCH`, derived in CI at release. See [`RELEASING.md`](RELEASING.md). |
+| Versioning | committed `versionName` is the source of truth; a bump reaching `main` triggers the release and the pipeline mints the `vX.Y.Z` tag. `versionCode = MAJOR*10000 + MINOR*100 + PATCH`. See [`RELEASING.md`](RELEASING.md). |
 | CI | Gitea workflows (`.gitea/workflows/ci.yaml`, `release.yaml`) |
 | Distribution | F-Droid (`fdroid-metadata/`) |