feat(service): Add working database persistency

lib/src/service/database.dart

@@ -0,0 +1,55 @@
+import 'package:anitrack/src/data/anime.dart';
+import 'package:sqflite/sqflite.dart';
+
+const animeTable = 'Anime';
+
+Future<void> _createDatabase(Database db, int version) async {
+  await db.execute(
+    '''
+    CREATE TABLE $animeTable(
+      id TEXT NOT NULL PRIMARY KEY,
+      state INTEGER NOT NULL,
+      episodesTotal INTEGER,
+      episodesWatched INTEGER NOT NULL,
+      thumbnailUrl TEXT NOT NULL,
+      title TEXT NOT NULL
+    )''',
+  );
+}
+
+class DatabaseService {
+  late final Database _db;
+
+  Future<void> initialize() async {
+    _db = await openDatabase(
+      'anitrack.db',
+      version: 1,
+      onCreate: _createDatabase,
+    );
+  }
+
+  Future<List<AnimeTrackingData>> loadAnimes() async {
+    final animes = await _db.query(animeTable);
+
+    return animes
+      .cast<Map<String, dynamic>>()
+      .map((Map<String, dynamic> anime) => AnimeTrackingData.fromJson(anime))
+      .toList();
+  }
+  
+  Future<void> addAnime(AnimeTrackingData data) async {
+    await _db.insert(
+      animeTable,
+      data.toJson(),
+    );
+  }
+
+  Future<void> updateAnime(AnimeTrackingData data) async {
+    await _db.update(
+      animeTable,
+      data.toJson(),
+      where: 'id = ?',
+      whereArgs: [data.id],
+    );
+  }
+}