Reduce request volume in the discovery helpers while keeping the source-priority model intact.
- make cache_discovery_recent_albums source-priority aware
- cap Spotify artist-album pagination in the discovery and incremental paths
- reduce the similar-artist sample size for the cache-refresh helper
- keep Spotify strict where fallback would contaminate source-specific IDs
- add regression coverage for source order, strict Spotify lookups, and pagination caps
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