feat: build and sign Android releases on tag push via Forgejo

Reads the release keystore from env vars in CI, falling back to the
debug keys locally. Adds fastlane metadata for F-Droid/Obtainium and
a .forgejo workflow that builds, signs, and publishes the APK on
pushing a vX.Y.Z+N tag.

app/fastlane/metadata/android/en-US/full_description.txt

@@ -0,0 +1,7 @@
+The Mews Browser is a reader for Mews pages, Gemini capsules, and Spartan
+sites. It's local-first: pages, bookmarks, subscriptions, and reading
+settings live in a local database, and network access only revalidates the
+cache.
+
+The reader controls presentation, not the page author, so every site reads
+consistently in the typeface, size, and color you choose.