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>

.gitea/workflows/ci.yaml

@@ -1,18 +1,23 @@
 name: CI
 
+# One gate per pull request. Branch pushes no longer trigger CI on their own,
+# so a change is built once on its PR (covering feature -> release/* and
+# release/* -> main) instead of once per push and again on the merge to main.
+# The merge itself is handled by release.yaml, which only does heavy work when
+# the merge actually cuts a release.
 on:
-  push:
-    branches:
-      - '**'
-    tags-ignore:
-      - '**'
+  pull_request:
 
-# Cancel superseded runs on the same branch.
+# Cancel superseded runs for the same PR.
 concurrency:
   group: ci-${{ github.ref }}
   cancel-in-progress: true
 
 jobs:
+  # Single job named `ci` so the required "CI" status check is always reported,
+  # even for docs-only PRs: those just skip the Android build and the job still
+  # succeeds (fast green check) instead of being filtered out and leaving the
+  # required check pending forever.
   ci:
     runs-on: docker
     env:
@@ -22,15 +27,53 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
         with:
+          # Full history so the base..HEAD diff below has a merge-base.
+          fetch-depth: 0
           submodules: recursive
 
+      # Cheap, always-on guard: the release build must stay reproducible for the
+      # official F-Droid repo (no AGP VCS-info embedding). Runs regardless of
+      # change scope so a regression can't slip through on a "docs-only" PR.
+      - name: Reproducible-release invariant
+        run: bash scripts/check_reproducible_release.sh
+
+      # Decide whether anything that affects the app build changed. Docs,
+      # F-Droid metadata and the licence don't, so those PRs skip the SDK +
+      # Gradle work below but still report a green `ci`.
+      - name: Classify change scope
+        id: scope
+        run: |
+          set -e
+          BASE="${{ github.base_ref }}"
+          # Full (not --depth=1) base fetch so the merge-base is present even when
+          # the PR branch forked several commits back; a shallow tip has no merge
+          # base with a divergent branch and `git diff base...HEAD` aborts.
+          git fetch --no-tags origin "$BASE"
+          MB=$(git merge-base "origin/$BASE" HEAD 2>/dev/null || true)
+          if [ -z "$MB" ]; then
+            # No common ancestor available — don't risk skipping the build.
+            echo "No merge base with origin/$BASE — running the full build to be safe."
+            echo "code=true" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          CHANGED=$(git diff --name-only "$MB" HEAD)
+          echo "Changed files:"; echo "$CHANGED"
+          if echo "$CHANGED" | grep -vE '(\.md$|^docs/|^fdroid-metadata/|^fastlane/|^LICENSE$)' | grep -q .; then
+            echo "code=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "code=false" >> "$GITHUB_OUTPUT"
+            echo "Docs/metadata-only change — skipping the Android build."
+          fi
+
       - name: Setup Java
+        if: steps.scope.outputs.code == 'true'
         uses: actions/setup-java@v4
         with:
           distribution: 'zulu'
           java-version: '17'
 
       - name: Setup Android SDK
+        if: steps.scope.outputs.code == 'true'
         uses: android-actions/setup-android@v3
         with:
           # Default ("tools platform-tools") drags in the Android Emulator
@@ -38,12 +81,14 @@ jobs:
           packages: ''
 
       - name: Setup Android SDK cache
+        if: steps.scope.outputs.code == 'true'
         uses: actions/cache@v4
         with:
           path: /opt/android-sdk
           key: ${{ runner.os }}-android-sdk-37-36.0.0
 
       - name: Install Android SDK packages
+        if: steps.scope.outputs.code == 'true'
         run: |
           yes | sdkmanager --licenses >/dev/null || true
           sdkmanager \
@@ -52,6 +97,7 @@ jobs:
             "build-tools;36.0.0"
 
       - name: Setup Gradle cache
+        if: steps.scope.outputs.code == 'true'
         uses: actions/cache@v4
         with:
           path: |
@@ -62,21 +108,25 @@ jobs:
             ${{ runner.os }}-gradle-
 
       - name: Grant execute permission for gradlew
+        if: steps.scope.outputs.code == 'true'
         run: chmod +x ./gradlew
 
       # No --no-daemon: the daemon lives only as long as this job container
       # and lets the following steps skip JVM startup + reconfiguration.
       - name: Lint (debug variant only)
+        if: steps.scope.outputs.code == 'true'
         run: ./gradlew lintDebug
 
       - name: Unit tests
+        if: steps.scope.outputs.code == 'true'
         run: ./gradlew testDebugUnitTest
 
       - name: Assemble debug APK
+        if: steps.scope.outputs.code == 'true'
         run: ./gradlew assembleDebug
 
       - name: Trivy filesystem scan
-        if: github.ref == 'refs/heads/main'
+        if: steps.scope.outputs.code == 'true'
         run: |
           set -e
           SUDO=""