feat(taste): era/decade taste facet — #1530

Milestone #160 Opt 2 (era half). A third taste facet alongside artists
+ genre tags: signed weights over decade buckets ("1990s") derived from
albums.release_date, rebuilt daily and scored into the taste match.

- Migration 0045: taste_profile_eras table (mirrors taste_profile_tags)
  + taste_tuning.era_scale column (DEFAULT 0.5).
- Build side (internal/taste): Config.EraScale ([0,1] damper, mirrors
  EnrichedTagScale), accumulate folds each play/like's decade at
  base*EraScale, persist atomic-replaces the era rows.
- Scorer (internal/recommendation): TasteProfile gains an era term (own
  tanh scale + additive 0.15 share so it never weakens the existing
  artist/tag signal when a track is undated); candidate queries return
  album release_date; decadeOf mirrors the builder helper.
- Tuning lab: era_scale threaded through recsettings + admin API + web
  card (auto-renders the new row) + Go/web tests.

Mood facet deferred to #1534 (partial enrichment coverage + needs
candidate-side enriched-tag loading).

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

web/src/lib/api/tuning.ts

@@ -21,6 +21,7 @@ export type TasteTuning = {
   engagement_neutral: number;
   engagement_full: number;
   enriched_tag_scale: number;
+  era_scale: number;
 };
 
 export type TuningScope = 'radio' | 'daily_mix' | 'taste';

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

@@ -37,7 +37,8 @@
     { key: 'engagement_hard_skip', label: 'Hard-skip point',  hint: 'Completion at/below which a play reads −1.' },
     { 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: '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.' }
   ];
 
   const profileScopes: { scope: 'radio' | 'daily_mix'; label: string; blurb: string }[] = [

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

@@ -37,6 +37,7 @@ const taste = (over: Partial<Record<string, number>> = {}) => ({
   engagement_neutral: 0.3,
   engagement_full: 0.9,
   enriched_tag_scale: 0.5,
+  era_scale: 0.5,
   ...over
 });