Make discovery pool population respect provider priority while keeping Spotify strict, and reduce unnecessary request volume in the hot discovery paths.
- keep discovery fan-out source-priority aware
- preserve cache use where freshness is not required
- cap Spotify artist-album pagination in discovery and cache refresh paths
- keep incremental release checks to a single page, since they only need the newest releases
- add regression coverage for provider order, strict Spotify handling, and pagination caps
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