Release builds were signed with the debug key, which Play rejects. Load
an upload keystore from android/key.properties (gitignored) and use it
for the release signing config, falling back to the debug key when the
file is absent so `flutter run --release` still works without a keystore.
Add key.properties.example documenting the keytool setup and a
build_android_release.sh helper that builds the signed AAB and refuses
to run when key.properties is missing.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
