ci(release): P1 hardening — versioning, F-Droid changelogs, R8 mapping, docs

P1.3 Versioning: the git tag is already the de-facto single source of truth
(every published versionCode uses MAJOR*10000+MINOR*100+PATCH; committed 13
was a stale outlier). Align the committed default to 20000 and document the
scheme in a comment + docs/RELEASING.md.

P1.4 F-Droid changelogs: a tag-only step extracts the tag's CHANGELOG section
into metadata/.../en-US/changelogs/<versionCode>.txt so clients show a
per-version "What's New". Also upload metadata/ (non-secret, never web-served)
alongside repo/ so changelog history survives across releases.

P1.5 R8 mapping: attach mapping-<version>.txt.gz to the Gitea release
(best-effort, continue-on-error) so user crash stacktraces stay
deobfuscatable. The gitea-release notes step is now an upsert (PATCH if the
release already exists) so it composes with the mapping step creating the
release first.

P1.6 docs/RELEASING.md: release ritual, versioning scheme, secrets inventory,
key custody/recovery, manual re-sign path, F-Droid repo details.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -23,7 +23,12 @@ android {
         applicationId = "de.jeanlucmakiola.calendula"
         minSdk = 29
         targetSdk = 36
-        versionCode = 13
+        // The git tag is the single source of truth for released builds: at
+        // release time .gitea/workflows/release.yaml derives both fields from
+        // the tag, with versionCode = MAJOR*10000 + MINOR*100 + PATCH
+        // (e.g. v2.0.0 -> 20000). These committed values are the dev/local
+        // default; keep them matching the latest released tag. See docs/RELEASING.md.
+        versionCode = 20000
         versionName = "2.0.0"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"