feat: pluggable discovery sources for richer album mosaic

[cdtinney]

Summary

Introduces a DiscoverySource interface so album discovery strategies are pluggable. The monolithic pipeline in getCurrentlyPlayingRelatedAlbums is replaced with a source registry — adding a new discovery strategy means implementing one interface and adding it to an array.

Three sources ship with this PR:

• Related artists (priority 10) — Last.fm + ListenBrainz, now querying all song artists instead of just the primary
• Genre fallback (priority 20) — searches Spotify for artists in the same genres when the related artist pool is small
• 2-hop traversal (priority 30) — fetches related artists of the top 5 related artists for deeper graph exploration

Also includes:

• Song-level discoveryResultCache (4h TTL) to skip the pipeline on repeated polls
• relatedArtistsCache TTL increased from 1h to 4h
• resolveArtistId extracted to its own module for reuse across sources
• Spotify recommendations API intentionally excluded (deprecated Nov 2024)

Testing

• 49 server tests pass (14 test files), including new tests for runDiscovery, each source, and the updated pipeline
• pnpm format:check and pnpm lint pass (warnings only, no errors)
• Manual testing needed: play a collaborative track and a niche genre track to verify mosaic variety