Add database migration from legacy data files

- Create DataMigrationService for migrating data files to SQLite
- Add sync database methods to AnimeSeriesService
- Update SerieScanner to save to database with file fallback
- Update anime API endpoints to use database with fallback
- Add delete endpoint for anime series
- Add automatic migration on startup in fastapi_app.py lifespan
- Add 28 unit tests for migration service
- Add 14 integration tests for migration flow
- Update infrastructure.md and database README docs

Migration runs automatically on startup, legacy data files preserved.

docs/infrastructure.md

@@ -162,7 +162,83 @@ All series-related WebSocket events include `key` as the primary identifier in t
 
 -   `AnimeSeriesService.get_by_key(key)` - **Primary lookup method**
 -   `AnimeSeriesService.get_by_id(id)` - Internal lookup by database ID
--   No `get_by_folder()` method exists - folder is never used for lookups
+-   `AnimeSeriesService.get_all(db)` - Get all series from database
+-   `AnimeSeriesService.create(db, key, name, site, folder, episode_dict)` - Create new series
+-   `AnimeSeriesService.update(db, id, **kwargs)` - Update existing series
+-   `AnimeSeriesService.delete(db, id)` - Delete series by ID
+-   `AnimeSeriesService.upsert_sync(db, key, name, site, folder, episode_dict)` - Sync upsert for scanner
+
+No `get_by_folder()` method exists - folder is never used for lookups.
+
+## Data Storage Migration
+
+### Background
+
+The application has migrated from file-based storage to SQLite database storage for anime series metadata.
+
+**Previous Storage (Deprecated)**:
+- Individual `data` files (no extension) in each anime folder
+- Example: `/anime-directory/Attack on Titan (2013)/data`
+
+**Current Storage (Database)**:
+- SQLite database at `data/aniworld.db`
+- Managed by `AnimeSeriesService` using SQLAlchemy
+
+### Migration Service
+
+The `DataMigrationService` handles automatic migration of legacy data files to the database:
+
+```python
+from src.server.services.data_migration_service import DataMigrationService
+
+# Check for legacy files
+service = DataMigrationService()
+files = await service.check_for_legacy_data_files(anime_directory)
+
+# Migrate all to database
+result = await service.migrate_all_legacy_data(anime_directory, db_session)
+print(result)  # Migration Result: 10 migrated, 2 skipped, 0 failed
+
+# Optional: cleanup old files with backup
+await service.cleanup_migrated_files(files, backup=True)
+```
+
+### Automatic Migration on Startup
+
+Migration runs automatically during application startup:
+
+1. Database is initialized (`init_db()`)
+2. Legacy data files are detected
+3. Files are migrated to database
+4. Results are logged (no files are deleted automatically)
+
+### Migration Result
+
+```python
+@dataclass
+class MigrationResult:
+    total_found: int    # Total legacy files found
+    migrated: int       # Successfully migrated
+    failed: int         # Failed to migrate
+    skipped: int        # Already in database
+    errors: List[str]   # Error messages
+```
+
+### Deprecation Notes
+
+- **Legacy file-based storage is deprecated** - Do not create new `data` files
+- **SerieScanner**: Updated to save to database (with file fallback for CLI)
+- **API endpoints**: Now use database as primary source
+- **CLI**: Still uses file-based storage for backward compatibility
+
+### Related Files
+
+| File | Purpose |
+| ---- | ------- |
+| `src/server/services/data_migration_service.py` | Migration service |
+| `src/server/database/service.py` | Database CRUD operations |
+| `src/server/database/models.py` | SQLAlchemy models |
+| `src/core/SerieScanner.py` | Scanner with DB support |
 
 ## Core Services