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/fastapi_app.py

@@ -186,7 +186,34 @@ async def lifespan(_application: FastAPI):
         # Subscribe to progress events
         progress_service.subscribe("progress_updated", progress_event_handler)
 
-        # Sync series from data files to database FIRST (one-time setup)
+        # Check if initial setup has been completed
+        try:
+            from src.server.database.connection import get_db_session
+            from src.server.database.system_settings_service import (
+                SystemSettingsService,
+            )
+            
+            async with get_db_session() as db:
+                is_initial_scan_done = (
+                    await SystemSettingsService.is_initial_scan_completed(db)
+                )
+                
+            if is_initial_scan_done:
+                logger.info(
+                    "Initial scan already completed, skipping data file sync"
+                )
+            else:
+                logger.info(
+                    "Initial scan not completed, "
+                    "performing first-time setup"
+                )
+        except Exception as e:
+            logger.warning(
+                "Failed to check system settings: %s, assuming first run", e
+            )
+            is_initial_scan_done = False
+
+        # Sync series from data files to database (only on first run)
         # This must happen before SeriesApp initialization
         try:
             logger.info(
@@ -195,13 +222,33 @@ async def lifespan(_application: FastAPI):
             )
             
             if settings.anime_directory:
-                # Sync series from data files to database (one-time setup)
-                sync_count = await sync_series_from_data_files(
-                    settings.anime_directory
-                )
-                logger.info(
-                    "Data file sync complete. Added %d series.", sync_count
-                )
+                # Only sync from data files on first run
+                if not is_initial_scan_done:
+                    logger.info("Performing initial anime folder scan...")
+                    sync_count = await sync_series_from_data_files(
+                        settings.anime_directory
+                    )
+                    logger.info(
+                        "Data file sync complete. Added %d series.", sync_count
+                    )
+                    
+                    # Mark initial scan as completed
+                    try:
+                        async with get_db_session() as db:
+                            await (
+                                SystemSettingsService
+                                .mark_initial_scan_completed(db)
+                            )
+                        logger.info("Marked initial scan as completed")
+                    except Exception as e:
+                        logger.warning(
+                            "Failed to mark initial scan as completed: %s", e
+                        )
+                else:
+                    logger.info(
+                        "Skipping initial scan - "
+                        "already completed on previous run"
+                    )
                 
                 # Load series from database into SeriesApp's in-memory cache
                 from src.server.utils.dependencies import get_anime_service