refactor: Implement Singleton pattern for DatabaseManager

Refactor the database handling to use a Singleton pattern for `DatabaseManager`.

- A single, module-level instance `db_manager` is created in `core/database.py` to ensure one database connection is used throughout the application.
- `MediaRepository` and `MainController` are updated to use this shared instance instead of creating their own.
- This simplifies dependency injection and prevents potential issues with multiple database connections.
- Also, update `review.md` to reflect the progress.

core/database.py

@@ -154,3 +154,6 @@ class DatabaseManager:
         cur = self.conn.cursor()
         cur.execute("DELETE FROM media WHERE color_id = ?", (color_id,))
         self.conn.commit()
+
+
+db_manager = DatabaseManager()