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

@@ -39,6 +39,7 @@ from src.server.database.models import (
     AnimeSeries,
     DownloadQueueItem,
     Episode,
+    SystemSettings,
     UserSession,
 )
 from src.server.database.service import (
@@ -47,6 +48,7 @@ from src.server.database.service import (
     EpisodeService,
     UserSessionService,
 )
+from src.server.database.system_settings_service import SystemSettingsService
 
 __all__ = [
     # Base and connection
@@ -69,10 +71,12 @@ __all__ = [
     "AnimeSeries",
     "Episode",
     "DownloadQueueItem",
+    "SystemSettings",
     "UserSession",
     # Services
     "AnimeSeriesService",
     "EpisodeService",
     "DownloadQueueService",
+    "SystemSettingsService",
     "UserSessionService",
 ]