fix(listenbrainz): similarity hits Labs API, not the 404'ing main API

Root cause of zero LB recommendations (every similar-recordings AND
similar-artists call returned HTTP 404, worker logs):

- Wrong host/path: client called
  api.listenbrainz.org/1/explore/similar-{recordings,artists}/{mbid}.
  /explore/... is a WEBSITE route, not an API endpoint — it 308s then
  404s. Similarity datasets live on the separate Labs API.
- Invalid algorithm: the hardcoded
  session_…_session_30_…_limit_100_filter_True_… is not a permitted
  Labs enum member (400s) regardless of host.

Verified against the live Labs API:
  GET labs.api.listenbrainz.org/similar-recordings/json
      ?recording_mbids=<mbid>&algorithm=<algo>
  GET labs.api.listenbrainz.org/similar-artists/json
      ?artist_mbids=<mbid>&algorithm=<algo>
  algorithm=session_based_days_9000_session_300_contribution_5_threshold_15_limit_50_skip_30
  → 200 for both. Response field names (recording_mbid/artist_mbid/
  name/score) already match the existing structs — parsing unchanged.

- Add defaultLabsBaseURL + Client.LabsBaseURL (separate from the main
  BaseURL; scrobble submission still uses api.listenbrainz.org).
- Drop the count/limit query param — result size is encoded in the
  algorithm name (limit_50); caller still applies its own top-K.
- Tests: newTestClient sets LabsBaseURL; the two *_LimitParamSet tests
  become *_MbidParamSet (assert the Labs path + mbid query param).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/similarity/worker.go

@@ -1,9 +1,9 @@
 // Package similarity owns the inbound ListenBrainz similarity ingest
-// pipeline. A periodic worker queries LB's /explore/similar-recordings
-// and /explore/similar-artists endpoints for tracks the user has played,
-// filters returned MBIDs to the local library, and stores the top-K
-// edges in track_similarity / artist_similarity for M4c's radio
-// candidate-pool builder.
+// pipeline. A periodic worker queries the LB Labs API
+// (labs.api.listenbrainz.org similar-recordings / similar-artists) for
+// tracks the user has played, filters returned MBIDs to the local
+// library, and stores the top-K edges in track_similarity /
+// artist_similarity for M4c's radio candidate-pool builder.
 package similarity
 
 import (