chore: Add deprecation warnings and update documentation (Task 9)

Task 9: Clean up legacy code
- Added deprecation warnings to Serie.save_to_file() and load_from_file()
- Updated infrastructure.md with Data Storage section documenting:
  - SQLite database as primary storage
  - Legacy file storage as deprecated
  - Data migration process
- Added deprecation warning tests for Serie class
- Updated existing tests to handle new warnings
- All 1012 tests pass (872 unit + 55 API + 85 integration)

docs/infrastructure.md

@@ -164,6 +164,53 @@ All series-related WebSocket events include `key` as the primary identifier in t
 -   `AnimeSeriesService.get_by_id(id)` - Internal lookup by database ID
 -   No `get_by_folder()` method exists - folder is never used for lookups
 
+## Data Storage
+
+### Storage Architecture
+
+The application uses **SQLite database** as the primary storage for anime series metadata. This replaces the legacy file-based storage system.
+
+| Storage Method | Status                | Location                  | Purpose                        |
+| -------------- | --------------------- | ------------------------- | ------------------------------ |
+| SQLite DB      | **Primary (Current)** | `data/aniworld.db`        | All series metadata and state  |
+| Data Files     | **Deprecated**        | `{anime_dir}/*/data`      | Legacy per-series JSON files   |
+
+### Database Storage (Recommended)
+
+All new series are stored in the SQLite database via `AnimeSeriesService`:
+
+```python
+# Add series to database
+await AnimeSeriesService.create(db_session, series_data)
+
+# Query series by key
+series = await AnimeSeriesService.get_by_key(db_session, "attack-on-titan")
+
+# Update series
+await AnimeSeriesService.update(db_session, series_id, update_data)
+```
+
+### Legacy File Storage (Deprecated)
+
+The legacy file-based storage is **deprecated** and will be removed in v3.0.0:
+
+- `Serie.save_to_file()` - Deprecated, use `AnimeSeriesService.create()`
+- `Serie.load_from_file()` - Deprecated, use `AnimeSeriesService.get_by_key()`
+- `SerieList.add()` - Deprecated, use `SerieList.add_to_db()`
+
+Deprecation warnings are raised when using these methods.
+
+### Data Migration
+
+On application startup, the system automatically migrates legacy data files to the database:
+
+1. **Scan**: `DataMigrationService.scan_for_data_files()` finds legacy `data` files
+2. **Migrate**: `DataMigrationService.migrate_data_file()` imports each file to DB
+3. **Skip**: Existing series (by key) are skipped; changed episode data is updated
+4. **Log**: Migration results are logged at startup
+
+Migration is idempotent and safe to run multiple times.
+
 ## Core Services
 
 ### SeriesApp (`src/core/SeriesApp.py`)