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.

docs/instructions.md

@@ -119,23 +119,18 @@ For each task completed:
 
 ## TODO List:
 
-✅ **FIXED:** Anime list endpoint now correctly returns anime data after server startup.
+Make sure you do not produce doublicate code. the function below is mostly implemented.
+make sure you maintain the function on one location
 
-**Root Cause:** The anime list was empty because:
-1. The `SeriesApp.list` was initialized with `skip_load=True` to avoid loading from filesystem during initialization
-2. Series data is synced from filesystem data files to the database during server startup
-3. Series are then loaded from the database into `SeriesApp` memory via `anime_service._load_series_from_db()`
-4. The server needed to be restarted to complete this initialization process
+1. scanning anime from folder
+   make sure that scanning anime from folder only runs on setup and not on each start
 
-**Solution:** The existing startup process in [fastapi_app.py](../src/server/fastapi_app.py) correctly:
-- Syncs series from data files to database via `sync_series_from_data_files()`
-- Loads series from database into memory via `anime_service._load_series_from_db()`
+2. Nfo scan
+   make sure nfo scan runs only on setup and not on each start
 
-The issue was resolved by restarting the server to allow the full initialization process to complete.
+3. nfo data
+   during nfo scan read tmdb id from nfo file and write it in db.
+   during nfo scan read tvdb id from nfo file and write it in db.
 
-**Verified:** GET `/api/anime` now returns 192 anime series with complete metadata including:
-- Unique key (primary identifier)
-- Name and folder
-- Missing episodes tracking  
-- NFO metadata status
-- TMDB/TVDB IDs when available
+4. Media scan
+   make sure media scan runs only on setup and not on each start