Implement initial scan tracking for one-time setup

- Add SystemSettings model to track setup completion status
- Create SystemSettingsService for managing setup flags
- Modify fastapi_app startup to check and set initial_scan_completed flag
- Anime folder scanning now only runs on first startup
- Update DATABASE.md with new system_settings table documentation
- Add unit test for SystemSettingsService functionality

This ensures expensive one-time operations like scanning the entire anime
directory only occur during initial setup, not on every application restart.

src/server/database/init.py

@@ -36,6 +36,7 @@ EXPECTED_TABLES = {
     "episodes",
     "download_queue",
     "user_sessions",
+    "system_settings",
 }
 
 # Expected indexes for performance