feat(indexers): send an indexer's grabs to a chosen download client

[bbutlerau]

Summary

Pullbox sends every torrent grab to the highest-priority torrent client, and every Usenet grab to the highest-priority Usenet client. There is no way to keep one indexer's releases in a particular client.

That matters for private trackers. A common setup is a public-tracker qBittorrent behind a VPN, plus a second qBittorrent outside it whose IP is registered with a private tracker and which follows that tracker's seeding rules. Today, releases from the private tracker land in whichever client has the higher priority.

This adds a per-indexer "Download client" setting, like Sonarr's. Leave it at the default and nothing changes; pick a client and every grab from that indexer goes there.

Related Issues

None filed.

Changes

• indexer_configs.download_client_id: nullable foreign key to download_client_configs, ON DELETE SET NULL. Migration i6c7d8e9f012 adds it, and existing indexers stay unpinned.
• DownloadService.send_to_client uses the indexer's pinned client when it has one, and the existing priority-based selection otherwise.
• A pinned client is never swapped for another one. If it is disabled, or cannot take the release's protocol, the grab fails with an error that names the indexer and the client, instead of quietly sending a private-tracker torrent somewhere else.
• Retries and status polling need no change, because they already follow download_client_config_id on the download record.
• Indexer API: download_client_id on create, update and response. The client must exist and take the indexer's protocol: Torznab takes torrent clients, Newznab takes Usenet clients. Sending null clears the pin.
• Prowlarr and Jackett sync don't touch the field, so a pin survives a re-sync.
• More than one client of a type is now allowed (second commit). The add-client picker greyed out a type once one existed and the API rejected it, which made pinning useless for the main case: two qBittorrents. AirDC++ already allowed several. Status polling and completed-download path mapping now use the exact client recorded on the download (download_client_config_id) rather than the first client of that type, falling back to the type only for older rows without an ID. Otherwise the second qBittorrent's downloads would be polled and path-mapped through the first one.
• Settings → Indexers: a "Download client" dropdown under Advanced Settings that lists only clients of the indexer's protocol. Indexer cards show a "Client: name" pill when an indexer is pinned.

Checklist

• Tests pass locally. New: tests/api/test_indexer_download_client.py, repeat-type cases in tests/tasks/test_download_monitor_poll.py and test_download_post_processing_sources.py, the clients API tests (a second same-type client is now accepted), TestIndexerPinnedClient in tests/integration/test_download_clients_integration.py, a migration round-trip in tests/integration/test_migrations.py, and a settings render test in tests/ui/test_settings_shell_ui_routes.py; the new tests fail on develop without the change. Full non-browser suite: 14,826 passed; 13 failed, all needing tools my test container lacks (ps, Docker smoke, release tooling), none in code this PR touches.
• Lint clean (ruff check src/ tests/)
• Type check clean for the touched files (mypy --strict on download_service.py, api/v1/indexers.py, ui/settings_routes.py, models/indexer.py, schemas/indexer.py)
• Format clean (ruff format --check src/ tests/ alembic/)
• New code has test coverage
• Documentation updated: the indexer settings docs on pullbox.app would need a line about the new dropdown
• Commit messages use documented conventional prefixes
• CHANGELOG.md updated: left for release prep, per the contributing guide

Screenshots

The new field sits under Advanced Settings in the indexer dialog, below Priority. I haven't attached screenshots.

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