refactor: app metadata (#28113)

* refactor: app metadata

* refactor to per row store

* cleanup

* more test

* review changes

* more refactor

* refactor

---------

Co-authored-by: shenlong-tanwen <139912620+shalong-tanwen@users.noreply.github.com>

mobile/test/unit/repositories/metadata_repository_test.dart

@@ -0,0 +1,24 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:immich_mobile/domain/models/metadata_key.dart';
+
+void main() {
+  group('MetadataKey', () {
+    test('every key round-trips its default value losslessly', () {
+      for (final key in MetadataKey.values) {
+        final encoded = key.encode(key.defaultValue);
+        final decoded = key.decode(encoded);
+        expect(decoded, key.defaultValue, reason: 'round-trip failed for ${key.name}');
+      }
+    });
+
+    test('decode falls back to the default value when the raw input is unparseable', () {
+      for (final key in MetadataKey.values) {
+        expect(
+          key.decode('not a valid encoding for any key'),
+          key.defaultValue,
+          reason: 'fallback failed for ${key.name}',
+        );
+      }
+    });
+  });
+}