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/fastlane_to_fdroid_localized.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+# Single source of truth: fastlane/metadata/android/<locale>/ feeds BOTH the
+# official F-Droid repo (harvested from source automatically) and the
+# self-hosted repo. This script transforms the fastlane layout into the F-Droid
+# "localized" layout that the self-hosted `fdroid update` consumes, so we don't
+# maintain two copies.
+#
+#   usage: fastlane_to_fdroid_localized.sh <fastlane_android_dir> <out_localized_dir>
+#   e.g.   scripts/fastlane_to_fdroid_localized.sh \
+#              fastlane/metadata/android \
+#              fdroid/metadata/de.jeanlucmakiola.agendula
+#
+# Mapping (fastlane -> F-Droid repo localized):
+#   short_description.txt        -> summary.txt
+#   full_description.txt         -> description.txt
+#   title.txt                    -> name.txt
+#   images/icon.png              -> icon.png
+#   images/phoneScreenshots/*    -> phoneScreenshots/*
+#   changelogs/<versionCode>.txt -> changelogs/<versionCode>.txt
+# (changelogs are seeded into the fastlane tree by
+#  scripts/sync_changelog_to_fastlane.sh.)
+set -euo pipefail
+
+SRC="${1:?need fastlane android dir, e.g. fastlane/metadata/android}"
+OUT="${2:?need output localized dir, e.g. fdroid/metadata/<appid>}"
+
+shopt -s nullglob
+for locdir in "$SRC"/*/; do
+  loc="$(basename "$locdir")"
+  dst="$OUT/$loc"
+  mkdir -p "$dst"
+  [ -f "$locdir/short_description.txt" ] && cp "$locdir/short_description.txt" "$dst/summary.txt"
+  [ -f "$locdir/full_description.txt"  ] && cp "$locdir/full_description.txt"  "$dst/description.txt"
+  [ -f "$locdir/title.txt"             ] && cp "$locdir/title.txt"             "$dst/name.txt"
+  [ -f "$locdir/images/icon.png"       ] && cp "$locdir/images/icon.png"       "$dst/icon.png"
+  if [ -d "$locdir/images/phoneScreenshots" ]; then
+    mkdir -p "$dst/phoneScreenshots"
+    cp "$locdir"images/phoneScreenshots/* "$dst/phoneScreenshots/"
+  fi
+  # Per-version changelogs live in the same fastlane tree (see
+  # scripts/sync_changelog_to_fastlane.sh) and map straight across.
+  if [ -d "$locdir/changelogs" ]; then
+    mkdir -p "$dst/changelogs"
+    cp "$locdir"changelogs/* "$dst/changelogs/"
+  fi
+done
+
+echo "Built F-Droid localized metadata in '$OUT' from '$SRC'"