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>

CHANGELOG.md

@@ -1,8 +1,9 @@
 # Changelog
 
 All notable changes to this project are documented here. The format follows
-[Keep a Changelog](https://keepachangelog.com/); the latest released git tag is
-the source of truth for version codes (see Calendula's `docs/RELEASING.md`).
+[Keep a Changelog](https://keepachangelog.com/); the `versionName` committed in
+`app/build.gradle.kts` is the source of truth for a release (see
+`docs/RELEASING.md`), and the `vX.Y.Z` tag is minted by the pipeline.
 
 ## [Unreleased]