docs(plan): add M4b ListenBrainz inbound similarity ingest plan
8-task TDD-shaped plan covering: migration 0009 (track_similarity +
artist_similarity tables with multi-source PK), sqlc queries (list-
needing, get-by-mbids, upsert), LB client SimilarRecordings + Similar
Artists methods (7 httptest tests each), Worker skeleton + tickOnce
with track and artist passes (10 integration tests covering top-K=20,
7-day cap, in-library filter, 429-abort-tick, transient-skip, no-MBID
skip), main.go boot wiring, final verification.
bvandeusen
9ec9caf667