ci(release): beta releases as Codeberg-only pre-releases (#38)

### What this changes

Adds beta releases. Pushing a `release/*` branch whose committed `versionName` is `X.Y.Z-beta.N` makes the new `.gitea/workflows/beta.yaml` run the unit tests, build and sign the APK with the app key, and publish it as a **Codeberg pre-release** (APK + `.sha256`), plus a Gitea pre-release with the R8 mapping. F-Droid (self-hosted and official) and Play never get a beta; Obtainium only offers it with *Include prereleases* on.

- **`scripts/version_info.sh`** is the single source for `versionName` → `versionCode`, used by `release.yaml`, `beta.yaml`, the changelog sync and the store-listing check. From 1.1.0: `X*1000000 + Y*10000 + Z*100 + N` for betas (N = 1–98), `+ 99` for stable, so `1.1.0-beta.1` → `1010001`, `1.1.0` → `1010099`. All 1.0.x versions keep the legacy formula, so `release/v1.0.1` (code `10001`) stays valid.
- **Shared publish scripts:** `scripts/publish_codeberg_release.sh`, `scripts/publish_gitea_release.sh` and `scripts/release_notes.sh`, moved out of `release.yaml`. The stable path behaves as before.
- **Guards:**
  - CI fails a PR whose committed `versionCode` doesn't match its `versionName`.
  - CI fails a PR into `main` that carries a beta version.
  - `release.yaml`'s `detect` refuses a beta on `main` as a backstop.
  - `beta.yaml` refuses a beta of a version that has already shipped as stable.
  - Betas get no store What's New file.
- **Docs:** "Cutting a beta" and the versionCode table in `docs/RELEASING.md`; a note on beta tags in `docs/fdroid-official/README.md`; how to opt in to betas in the README.

### Why

To ship a test build of an upcoming version (e.g. 1.1.0) to opted-in testers before the stable release, without it reaching F-Droid or Play users.

### How it was tested

- `scripts/version_info.sh` against stable, beta, legacy and invalid version names.
- Both publish scripts against a mock forge API: create, re-run (PATCH plus asset replacement), Codeberg's 500-then-retry path, and the skip when no token is set.
- A scratch copy with `1.1.0-beta.1` committed: the version check passes, the changelog sync and `check_store_listing.py --complete` pass without a What's New, the PR-into-main guard trips, and a wrong `versionCode` is rejected.
- `sync_changelog_to_fastlane.sh` and `check_store_listing.py` (with and without `--complete`) still pass on the current `1.0.0`.
- All three workflow files parse as YAML.

Not run on the real runners yet. The first beta push is the live test of `beta.yaml`, which assumes a mirrored branch push starts a workflow on Gitea, the same way pushes to `main` already do.

### Checklist

- [x] No `versionName` / `versionCode` bump
- [x] No `values-*/strings.xml` touched
- [x] `CHANGELOG.md` not updated: this is release infrastructure, not a user-visible change

Co-authored-by: Jean-Luc Makiola <business@jeanlucmakiola.de>
Reviewed-on: https://codeberg.org/jlmakiola/agendula/pulls/38

app/build.gradle.kts

@@ -26,10 +26,12 @@ android {
         // These committed values ARE the source of truth for a release: merging
         // a bumped versionName into main triggers .gitea/workflows/release.yaml,
         // which builds this version and then creates the matching vX.Y.Z tag +
-        // release itself (versionCode is pinned to MAJOR*10000 + MINOR*100 +
-        // PATCH from versionName, e.g. 1.0.0 -> 10000). Releases were flagged as
-        // pre-releases while MAJOR was 0; 1.0.0 is the first stable one, and the
-        // pipeline graduates it on its own. See docs/RELEASING.md.
+        // release itself. A versionName of X.Y.Z-beta.N pushed to a release/*
+        // branch instead cuts a Codeberg-only pre-release (beta.yaml).
+        // versionCode is derived from versionName by scripts/version_info.sh
+        // (1.0.x: 1.0.0 -> 10000; from 1.1.0: 1.1.0-beta.1 -> 1010001,
+        // 1.1.0 -> 1010099), and CI fails if the committed one doesn't match.
+        // See docs/RELEASING.md.
         versionCode = 10000
         versionName = "1.0.0"