feat(taste): track_tags schema + enrichment queries (Opt 1 foundation)

First step of taste-profile fidelity via metadata enrichment (milestone
#160, task #1490) — no ML sidecar, operator's constraint.

The taste profile's tag facet is built purely from raw ID3 tracks.genre
(splitGenres in internal/taste/profile.go). This lands the data layer for
enriching it with track-level folksonomy tags:

- track_tags(track_id, tag, weight) — a global cache of style/mood tags,
  top-K per track, weight = normalized folksonomy strength [0,1].
- tracks.tag_source / tag_sources_version — versioned enrichment
  bookkeeping mirroring artists.artist_art_source (NULL = eligible,
  provider name = found, 'none' = settled, version bump = re-process).
- Queries: ListTracksMissingTags (batch drainer), DeleteTrackTags +
  InsertTrackTag (atomic per-track replace), SetTrackTagSource, and
  ListPlayed/LikedTrackTagsForUser for the recompute to union enriched
  tags into the tag facet alongside genre.

No consumer yet — the enricher (MusicBrainz + Last.fm providers,
track-level) and the taste-recompute integration land next.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/db/migrations/0042_track_tags.down.sql

@@ -0,0 +1,5 @@
+-- Reverse 0042_track_tags.up.sql.
+DROP INDEX IF EXISTS tracks_tag_source_idx;
+ALTER TABLE tracks DROP COLUMN IF EXISTS tag_sources_version;
+ALTER TABLE tracks DROP COLUMN IF EXISTS tag_source;
+DROP TABLE IF EXISTS track_tags;

internal/db/migrations/0042_track_tags.up.sql

@@ -0,0 +1,26 @@
+-- Track-level folksonomy tags for taste enrichment (milestone #160, #1490).
+--
+-- A global (not per-user) cache of style/mood tags fetched from
+-- MusicBrainz (keyless) + Last.fm (when a key is configured), keyed by
+-- track. Feeds the taste recompute's tag facet alongside raw ID3 genre,
+-- lifting resolution from coarse "Rock" to "post-punk / shoegaze /
+-- melancholic". The enricher caps to the top-K tags per track at write
+-- time so a heavily-tagged track can't dominate the profile. weight is a
+-- normalized folksonomy strength in [0,1]. Mirrors the coverart
+-- enricher's versioned source-tracking pattern.
+CREATE TABLE track_tags (
+    track_id uuid             NOT NULL REFERENCES tracks(id) ON DELETE CASCADE,
+    tag      text             NOT NULL,
+    weight   double precision NOT NULL DEFAULT 1,
+    PRIMARY KEY (track_id, tag)
+);
+CREATE INDEX track_tags_track_idx ON track_tags (track_id);
+
+-- Enrichment bookkeeping (mirror artists.artist_art_source):
+--   tag_source NULL                            → not yet processed (eligible)
+--   tag_source 'lastfm' / 'musicbrainz' / 'mixed' → found, cached
+--   tag_source 'none'                          → providers returned nothing
+--   tag_sources_version                        → bump to re-process none/stale
+ALTER TABLE tracks ADD COLUMN tag_source text;
+ALTER TABLE tracks ADD COLUMN tag_sources_version integer NOT NULL DEFAULT 0;
+CREATE INDEX tracks_tag_source_idx ON tracks (tag_source, tag_sources_version);