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()

core/media.py

@@ -1,13 +1,13 @@
 from pathlib import Path
 import shutil
-from core.database import DatabaseManager
+from core.database import db_manager
 
 MEDIA_DIR = Path("media")
 DEFAULT_ICON = Path("media/default_icon.jpg")
 
 class MediaRepository:
-    def __init__(self, db: DatabaseManager):
-        self.db = db
+    def __init__(self):
+        self.db = db_manager
 
     def sync_media(self):
         disk_colors = {d.name for d in MEDIA_DIR.iterdir() if d.is_dir()}