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.

tests/unit/test_system_settings_service.py

@@ -0,0 +1,58 @@
+"""Test the system settings service integration."""
+import pytest
+
+from src.server.database.connection import get_db_session, init_db
+from src.server.database.system_settings_service import SystemSettingsService
+
+
+@pytest.mark.asyncio
+async def test_system_settings_integration():
+    """Test SystemSettings service with actual database operations."""
+    # Initialize database
+    await init_db()
+    
+    # Test get_or_create (should create on first call)
+    async with get_db_session() as db:
+        settings = await SystemSettingsService.get_or_create(db)
+        assert settings is not None
+        assert settings.id is not None
+        assert settings.initial_scan_completed is False
+        assert settings.initial_nfo_scan_completed is False
+        assert settings.initial_media_scan_completed is False
+        
+    # Test checking individual flags
+    async with get_db_session() as db:
+        is_scan_done = await SystemSettingsService.is_initial_scan_completed(db)
+        assert is_scan_done is False
+        
+        is_nfo_done = await SystemSettingsService.is_initial_nfo_scan_completed(db)
+        assert is_nfo_done is False
+        
+        is_media_done = await SystemSettingsService.is_initial_media_scan_completed(db)
+        assert is_media_done is False
+    
+    # Test marking scans as completed
+    async with get_db_session() as db:
+        await SystemSettingsService.mark_initial_scan_completed(db)
+        
+    async with get_db_session() as db:
+        is_scan_done = await SystemSettingsService.is_initial_scan_completed(db)
+        assert is_scan_done is True
+        
+        # Others should still be False
+        is_nfo_done = await SystemSettingsService.is_initial_nfo_scan_completed(db)
+        assert is_nfo_done is False
+    
+    # Test reset
+    async with get_db_session() as db:
+        await SystemSettingsService.reset_all_scans(db)
+        
+    async with get_db_session() as db:
+        settings = await SystemSettingsService.get_or_create(db)
+        assert settings.initial_scan_completed is False
+        assert settings.initial_nfo_scan_completed is False
+        assert settings.initial_media_scan_completed is False
+
+
+if __name__ == "__main__":
+    pytest.main([__file__, "-v"])