ci: add signed release workflow and fastlane metadata

Adds a Forgejo Actions workflow that builds a release APK on tag push,
reading signing credentials from repo secrets with a debug-key fallback
for local builds, and publishes the APK plus fastlane changelog to a
Forgejo release.

android/app/build.gradle

@@ -32,11 +32,18 @@ android {
         targetSdkVersion 36
     }
 
+    signingConfigs {
+        release {
+            storeFile = System.getenv("KEYSTORE_PATH") ? file(System.getenv("KEYSTORE_PATH")) : null
+            storePassword = System.getenv("STORE_PASSWORD")
+            keyAlias = System.getenv("KEY_ALIAS")
+            keyPassword = System.getenv("KEY_PASSWORD")
+        }
+    }
+
     buildTypes {
         release {
-            // TODO: Add your own signing config for the release build.
-            // Signing with the debug keys for now, so `flutter run --release` works.
-            signingConfig = signingConfigs.debug
+            signingConfig = System.getenv("KEYSTORE_PATH") ? signingConfigs.release : signingConfigs.debug
         }
     }
 }