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.
Refactor the signal handling for the camera start/stop button in `MainController` to simplify logic and improve reliability.
- Replaced the dynamic connect/disconnect pattern with a single, persistent signal connection to `_on_start_button_clicked`.
- Centralized UI state updates (e.g., button text) into a new `_update_start_button_state` method.
- This eliminates potential errors from mismatched signal connections and makes the control flow easier to follow.
- Also fixes major indentation errors caused by a previous faulty replacement.
