Google Play is live: README + production track (#244)

Calendula is published on Google Play, so the pipeline and the docs stop treating it as pending.

**README**
- Google Play badge next to F-Droid/Obtainium, linking the store page.
- Install table: the Play row goes from "Coming soon / —" to a real channel.
- The "coming soon" section becomes a live one with the store link; the closed-testing "testers wanted" call drops out. The note that Play signs with Google's key (so switching channels needs an uninstall) stays, and the trailing shared-signing-key line now flags Play as the exception.

**Play track**
- Release uploads default to `production` instead of `internal` — in the lane (`fastlane/Fastfile`) and in the workflow default (`.gitea/workflows/release.yaml`). Merging a bumped versionName to main is already the human gate, so the manual Console promotion only added delay.
- `docs/RELEASING.md` updated to match, including the escape hatches (`PLAY_TRACK=internal` to stage, `PLAY_RELEASE_STATUS=draft` to hold).

Note: if a `PLAY_TRACK` repo variable is set on Gitea it still wins over the new default — needs checking there.

No issue for this one.

Co-authored-by: Jean-Luc Makiola <business@jeanlucmakiola.de>
Reviewed-on: https://codeberg.org/jlmakiola/calendula/pulls/244

fastlane/Fastfile

@@ -29,7 +29,7 @@ platform :android do
 
     supply(
       aab: aab,
-      track: options[:track] || "internal",
+      track: options[:track] || "production",
       release_status: options[:release_status] || "completed",
 
       # The APK belongs to F-Droid and the Codeberg download; Play only ever