Two architectural cleanups on top of the download engine refactor.
(1) Shared dataclasses move to neutral plugin package.
TrackResult, AlbumResult, DownloadStatus, SearchResult lived in
core/soulseek_client.py for historical reasons — every other plugin
imported them from the soulseek module just to satisfy the contract,
coupling 8 clients to a sibling source for type imports only. Moved
them to the new core/download_plugins/types.py module and updated all
14 import sites across the deezer/hifi/lidarr/qobuz/soundcloud/tidal/
youtube clients, the engine, matching engine, redownload helper, and
tests. Clean break, no backward-compat re-export.
(2) web_server.py boots the orchestrator via the singleton factory.
After construction it now calls set_download_orchestrator(...) so
get_download_orchestrator() returns the same instance the global
handle points at instead of lazily building a separate orchestrator.
Matches the get_metadata_engine() pattern.
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