feat: add Deluge download client support

[vavallee]

Closes #261

Summary

• New internal/downloader/deluge package: cookie-based JSON-RPC client for the Deluge Web UI (/json endpoint)
• Magnet links use core.add_torrent_magnet (hash returned directly); torrent URLs use web.download_torrent_from_url + web.add_torrents with poll-based hash resolution (same pattern as qBittorrent)
• Labels applied after add via the label plugin; errors ignored gracefully when plugin is not installed
• Stall detection: torrents in Error state
• Full adapter surface wired: TestClient, SendDownload, RemoveDownload, GetStalledIDs, GetLiveStatuses
• Frontend: Deluge added to client type dropdowns in both Add and Edit forms (port defaults to 8112, no username field, label plugin hint text)

Test plan

• go test ./internal/downloader/... — all pass (7 new Deluge tests covering login, add magnet, add URL, get torrents, remove, stall state)
• Add Deluge client in Settings UI — dropdown appears, port pre-fills to 8112, no username field shown, label hint visible
• Test connection button reaches Deluge and authenticates
• Grab a torrent release — download appears in Activity with live progress
• Remove a download — torrent removed from Deluge

[codecov]

Codecov Report

❌ Patch coverage is 43.91534% with 106 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/downloader/deluge/client.go	55.03%	42 Missing and 25 partials ⚠️
internal/downloader/adapter.go	2.50%	38 Missing and 1 partial ⚠️

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