refactor: extract shared mixins and helpers, drop dead abstractions

Introduces shared seams for paginated views, D-pad reorder, media control
routing, async singletons and the device method channel, then points the
open-coded copies at them.

Also removes unused models and duplicated provider/server plumbing, folds
the twice-implemented artifact store in the server, and factors the
repeated Flutter toolchain prologue in CI into a composite action.

test/database/app_database_test.dart

@@ -20,6 +20,7 @@ import 'package:plezy/utils/active_client_scope.dart';
 import 'package:plezy/utils/media_server_http_client.dart';
 import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 
+import '../test_helpers/download_fixtures.dart';
 import '../test_helpers/prefs.dart';
 
 void main() {
@@ -68,8 +69,8 @@ class _AppDatabaseTestSuite {
         // v20 dropped the profile_id FK so virtual Plex Home profiles can
         // persist join rows without a parent `profiles` row. Profile deletion
         // instead cleans up join rows explicitly (via the teardown flow's
-        // removeAllProfileConnectionsAndCleanup) before deleting the profile,
-        // so the cascade isn't needed.
+        // ProfileConnectionCleanup.removeAllProfileConnections) before deleting
+        // the profile, so the cascade isn't needed.
         final now = DateTime.now().millisecondsSinceEpoch;
         await db
             .into(db.connections)

test/database/download_operations_test.dart

@@ -8,6 +8,8 @@ import 'package:plezy/database/app_database.dart';
 import 'package:plezy/database/download_operations.dart';
 import 'package:plezy/models/download_models.dart';
 
+import '../test_helpers/download_fixtures.dart';
+
 void main() {
   late AppDatabase db;
 
@@ -19,115 +21,6 @@ void main() {
     await db.close();
   });
 
-  // ============================================================
-  // insertDownload
-  // ============================================================
-
-  group('insertDownload', () {
-    test('inserts a movie row with defaults', () async {
-      await db.insertDownload(
-        serverId: ServerId('srv'),
-        ratingKey: '100',
-        globalKey: 'srv:100',
-        type: 'movie',
-        status: DownloadStatus.queued.index,
-      );
-
-      final rows = await db.select(db.downloadedMedia).get();
-      expect(rows, hasLength(1));
-      final r = rows.first;
-      expect(r.serverId, 'srv');
-      expect(r.ratingKey, '100');
-      expect(r.globalKey, 'srv:100');
-      expect(r.type, 'movie');
-      expect(r.status, DownloadStatus.queued.index);
-      expect(r.parentRatingKey, isNull);
-      expect(r.grandparentRatingKey, isNull);
-      expect(r.mediaIndex, 0);
-    });
-
-    test('inserts an episode with parent and grandparent keys', () async {
-      await db.insertDownload(
-        serverId: ServerId('srv'),
-        ratingKey: 'ep1',
-        globalKey: 'srv:ep1',
-        type: 'episode',
-        parentRatingKey: 'season1',
-        grandparentRatingKey: 'show1',
-        status: DownloadStatus.queued.index,
-        mediaIndex: 7,
-      );
-
-      final row = (await db.select(db.downloadedMedia).get()).single;
-      expect(row.parentRatingKey, 'season1');
-      expect(row.grandparentRatingKey, 'show1');
-      expect(row.mediaIndex, 7);
-    });
-
-    test('atomically updates metadata and attempt state while preserving the row and physical fields', () async {
-      await db.insertDownload(
-        serverId: ServerId('srv'),
-        clientScopeId: 'scope-old',
-        ratingKey: '100',
-        globalKey: 'srv:100',
-        type: 'movie',
-        status: DownloadStatus.queued.index,
-        mediaIndex: 1,
-        mediaSourceId: 'source-old',
-      );
-      final original = (await db.getDownloadedMedia('srv:100'))!;
-      await (db.update(db.downloadedMedia)..where((row) => row.globalKey.equals('srv:100'))).write(
-        const DownloadedMediaCompanion(
-          progress: Value(50),
-          downloadedBytes: Value(500),
-          totalBytes: Value(1000),
-          videoFilePath: Value('downloads/video.mkv'),
-          safRootUri: Value('content://downloads'),
-          thumbPath: Value('downloads/thumb.jpg'),
-          downloadedAt: Value(1234),
-          errorMessage: Value('old error'),
-          retryCount: Value(2),
-          bgTaskId: Value('current-task'),
-        ),
-      );
-
-      await db.insertDownload(
-        serverId: ServerId('srv-new'),
-        clientScopeId: 'scope-new',
-        ratingKey: '100-new',
-        globalKey: 'srv:100',
-        type: 'episode',
-        parentRatingKey: 'season-new',
-        grandparentRatingKey: 'show-new',
-        status: DownloadStatus.failed.index,
-        mediaIndex: 3,
-        mediaSourceId: 'source-new',
-      );
-
-      final row = (await db.select(db.downloadedMedia).get()).single;
-      expect(row.id, original.id);
-      expect(row.serverId, 'srv-new');
-      expect(row.clientScopeId, 'scope-new');
-      expect(row.ratingKey, '100-new');
-      expect(row.type, 'episode');
-      expect(row.parentRatingKey, 'season-new');
-      expect(row.grandparentRatingKey, 'show-new');
-      expect(row.status, DownloadStatus.failed.index);
-      expect(row.mediaIndex, 3);
-      expect(row.mediaSourceId, 'source-new');
-      expect(row.progress, 0);
-      expect(row.downloadedBytes, 0);
-      expect(row.totalBytes, isNull);
-      expect(row.errorMessage, isNull);
-      expect(row.retryCount, 0);
-      expect(row.videoFilePath, 'downloads/video.mkv');
-      expect(row.safRootUri, 'content://downloads');
-      expect(row.thumbPath, 'downloads/thumb.jpg');
-      expect(row.downloadedAt, 1234);
-      expect(row.bgTaskId, 'current-task');
-    });
-  });
-
   group('insertQueuedDownload', () {
     test('atomically persists media identity, scope, policy, and queue state', () async {
       final tempDir = await Directory.systemTemp.createTemp('plezy_atomic_queue_');