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/library/mbids.go

@@ -29,8 +29,9 @@ func extractMBIDs(m tag.Metadata) (albumMBID, artistMBID string) {
 
 // extractRecordingMBID reads the MusicBrainz *recording* ID — Picard's
 // musicbrainz_recordingid, surfaced by dhowden/tag as mbz.Recording
-// (tag display name "MusicBrainz Track Id"). This is the id ListenBrainz
-// /explore/similar-recordings keys on and what tracks.mbid stores.
+// (tag display name "MusicBrainz Track Id"). This is the id the
+// ListenBrainz Labs similar-recordings API keys on and what tracks.mbid
+// stores.
 //
 // Deliberately NOT mbz.Track ("MusicBrainz Release Track Id"): that is
 // per-release-track, whereas similarity is per-recording. Kept separate