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>

scripts/sync_changelog_to_fastlane.sh

@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+# Write the current version's CHANGELOG.md section into the fastlane changelog
+# file that F-Droid harvests: fastlane/metadata/android/en-US/changelogs/<code>.txt
+# (en-US is F-Droid's fallback locale, so it covers every language).
+#
+# Run this when cutting a release (after editing CHANGELOG.md and bumping
+# versionName in app/build.gradle.kts) and COMMIT the result, so the OFFICIAL
+# F-Droid repo — which reads the changelog from the tagged source tree — shows
+# this version's "What's New". The self-hosted release pipeline also runs it so
+# its changelog never depends on the file having been committed. Idempotent.
+#
+# Extraction matches the awk used for the Gitea release notes so all three
+# (release notes, self-hosted changelog, official changelog) stay in sync.
+set -euo pipefail
+cd "$(dirname "$0")/.."   # repo root
+
+VERSION=$(grep -oP 'versionName\s*=\s*"\K[^"]+' app/build.gradle.kts)
+[ -n "$VERSION" ] || { echo "No versionName in app/build.gradle.kts" >&2; exit 1; }
+MAJOR=${VERSION%%.*}; rest=${VERSION#*.}; MINOR=${rest%%.*}; PATCH=${rest##*.}
+MAJOR=${MAJOR:-0}; MINOR=${MINOR:-0}; PATCH=${PATCH:-0}
+VERSION_CODE=$(( MAJOR * 10000 + MINOR * 100 + PATCH ))
+
+CL_DIR="fastlane/metadata/android/en-US/changelogs"
+mkdir -p "$CL_DIR"
+OUT="$CL_DIR/${VERSION_CODE}.txt"
+
+awk -v ver="$VERSION" '
+  $0 ~ "^## \\[" ver "\\]" { flag = 1; next }
+  /^## \[/ { flag = 0 }
+  flag' CHANGELOG.md > "$OUT"
+# Trim leading blank lines (same as the pipeline did).
+sed -i -e '/./,$!d' "$OUT"
+if [ ! -s "$OUT" ]; then
+  echo "See CHANGELOG.md for $VERSION." > "$OUT"
+fi
+
+CHARS=$(wc -m < "$OUT" | tr -d ' ')
+echo "Wrote $OUT (version $VERSION, code $VERSION_CODE, ${CHARS} chars)"
+if [ "$CHARS" -gt 500 ]; then
+  echo "  note: >500 chars — F-Droid may truncate this changelog in-client." >&2
+fi