Add torrent acquisition integrations

[TheusHen]

Summary

This adds a complete acquisition layer for self-hosted reading setups. Users can configure indexers and download clients, search releases, submit magnets or torrent URLs, and keep an auditable job history.

The server now supports qBittorrent, Transmission, Deluge, Prowlarr, Torznab, and Newznab. It also supports Readarr, Sonarr, Radarr, Lidarr, and Whisparr through their native command APIs, so those applications remain responsible for their own library and release-grab decisions.

Enabled acquisition rules are processed periodically. Rules can either select the best indexer result for a BitTorrent client or trigger a configured Arr command.

Implementation notes

• Added persistent endpoint, rule, and job models with an Alembic migration.
• Added authenticated acquisition endpoints under /v1/acquisition.
• Kept endpoint credentials server-side and excluded them from response schemas.
• Added API and service tests for the private endpoint contract.

Validation

• uv run ruff check on the changed Python files
• uv run mypy on the changed Python files
• Verified the new routes are registered in the FastAPI application

The integration test suite was not run locally because PostgreSQL/Docker was unavailable.