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>

web/src/lib/api/tuning.ts

@@ -23,6 +23,7 @@ export type TasteTuning = {
   engagement_full: number;
   enriched_tag_scale: number;
   era_scale: number;
+  mood_scale: number;
 };
 
 export type TuningScope = 'radio' | 'daily_mix' | 'taste';

web/src/routes/admin/tuning/+page.svelte

@@ -39,7 +39,8 @@
     { key: 'engagement_neutral',   label: 'Neutral point',    hint: 'Completion at which a play reads 0.' },
     { key: 'engagement_full',      label: 'Full point',       hint: 'Completion at/above which a play reads +1.' },
     { key: 'enriched_tag_scale',   label: 'Enriched tag weight', hint: 'How much folksonomy tags (MusicBrainz/Last.fm) count vs raw file genre, in [0, 1]. 0 = genre only.' },
-    { key: 'era_scale',            label: 'Era weight', hint: 'How strongly a decade-play imprints on the era facet, in [0, 1]. 0 = era ignored.' }
+    { key: 'era_scale',            label: 'Era weight', hint: 'How strongly a decade-play imprints on the era facet, in [0, 1]. 0 = era ignored.' },
+    { key: 'mood_scale',           label: 'Mood weight', hint: 'How strongly a mood-tagged play imprints on the mood facet (from folksonomy tags), in [0, 1]. 0 = mood ignored.' }
   ];
 
   const profileScopes: { scope: 'radio' | 'daily_mix'; label: string; blurb: string }[] = [

web/src/routes/admin/tuning/tuning.test.ts

@@ -39,6 +39,7 @@ const taste = (over: Partial<Record<string, number>> = {}) => ({
   engagement_full: 0.9,
   enriched_tag_scale: 0.5,
   era_scale: 0.5,
+  mood_scale: 0.5,
   ...over
 });