Fix multiple issues with the REST API that caused downloads to fail:
- Filter Click Sentinel.UNSET enum values from service parameter defaults that caused "Object of type Sentinel is not JSON serializable" errors
- Add missing select_titles and no_video args to dl.result() call
- Fix wanted param unpacking for list-tracks SeasonRange.parse_tokens()
- Add enum conversion for vcodec, range, sub_format, and export params that were passed as strings but expected as enums by dl.result()
- Add missing dl command params: split_audio, repack, imdb_id, output_dir, no_cache, reset_cache to DEFAULT_DOWNLOAD_PARAMS and download worker
- Expand vcodec/acodec/sub_format validation to cover all supported values
- Add POST /api/search endpoint for searching services by query
- Update Swagger docs with all new params and correct type definitions
- Add comprehensive REST API documentation (docs/API.md)
- Update ADVANCED_CONFIG.md with serve CLI options and API reference
