feat(taste): mood taste facet — #1534

Milestone #160 Opt 2b (mood half of the era+mood option). A fourth taste
facet alongside artists + genre tags + eras: signed weights over canonical
mood buckets (melancholic / energetic / chill / …) derived from a track's
enriched folksonomy tags (#1490).

- internal/mood: shared vocabulary — Of(tags) maps folksonomy tags to
  canonical mood buckets (synonyms collapse). Imported by both the taste
  builder and the scorer so a track's mood is derived identically.
- Migration 0047: taste_profile_moods table + taste_tuning.mood_scale
  (DEFAULT 0.5).
- Build side (internal/taste): Config.MoodScale ([0,1] damper, mirrors
  EraScale); accumulate folds each play/like's mood buckets at
  base*MoodScale; persist atomic-replaces the mood rows.
- Scorer (internal/recommendation): TasteProfile gains a mood term
  (own tanh scale + additive 0.12 share, so it never weakens the existing
  signal when a track has no mood tags). Match now takes the candidate's
  mood buckets; loaded per candidate (ListTrackTagsForTracks → mood.Of) in
  the primary similarity loader only — the near-whole-library fallback
  pool passes nil (mood → 0) to avoid a full-library tag scan.
- Tuning lab: mood_scale threaded through recsettings + admin API + web
  card ("Mood weight" row) + Go/web tests.

Coverage is partial (grows with tag enrichment; richer once Last.fm is
keyed), so mood is a supplement — neutral for tracks with no mood tags.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/db/migrations/0047_taste_profile_moods.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE taste_tuning DROP COLUMN IF EXISTS mood_scale;
+DROP TABLE IF EXISTS taste_profile_moods;

internal/db/migrations/0047_taste_profile_moods.up.sql

@@ -0,0 +1,25 @@
+-- 0047_taste_profile_moods.up.sql — mood taste facet (#1534, milestone #160
+-- Opt 2b). A fourth taste facet alongside artists + tags + eras: signed weights
+-- over canonical mood buckets (melancholic / energetic / chill / …) derived
+-- from a track's enriched folksonomy tags (#1490) via the internal/mood
+-- vocabulary, rebuilt daily. Weight is a signed float — positive = drawn to
+-- that mood, negative = passively avoided. Coverage is partial (grows with tag
+-- enrichment), so it's a supplement, not a foundation. Mirrors
+-- taste_profile_tags: CASCADE on user delete, indexed by (user_id, weight DESC).
+CREATE TABLE taste_profile_moods (
+    user_id    uuid             NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    mood       text             NOT NULL,
+    weight     double precision NOT NULL,
+    updated_at timestamptz      NOT NULL DEFAULT now(),
+    PRIMARY KEY (user_id, mood)
+);
+
+CREATE INDEX taste_profile_moods_user_weight_idx
+    ON taste_profile_moods (user_id, weight DESC);
+
+-- Mood-facet build knob, tunable in the lab (mirrors era_scale / enriched_tag_
+-- scale): scales how strongly a mood-tagged play imprints on the profile.
+-- DEFAULT 0.5 backfills the existing singleton and matches
+-- taste.DefaultConfig().MoodScale; 0 disables the mood facet entirely.
+ALTER TABLE taste_tuning
+    ADD COLUMN mood_scale double precision NOT NULL DEFAULT 0.5;