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>

internal/db/migrations/0045_taste_profile_eras.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE taste_tuning DROP COLUMN IF EXISTS era_scale;
+DROP TABLE IF EXISTS taste_profile_eras;

internal/db/migrations/0045_taste_profile_eras.up.sql

@@ -0,0 +1,26 @@
+-- 0045_taste_profile_eras.up.sql — era/decade taste facet (#1530,
+-- milestone #160 Opt 2). A third taste facet alongside artists + tags:
+-- signed weights over decade buckets ("1990s") derived from
+-- albums.release_date, rebuilt daily by internal/taste next to the
+-- artist/tag facets. Weight is a signed float — positive = drawn to
+-- that era, negative = passively avoided; magnitude reflects decayed
+-- engagement. Consumed by the recommendation scorer's taste match.
+-- Mirrors taste_profile_tags: CASCADE on user delete, indexed by
+-- (user_id, weight DESC) so "top eras" reads stay cheap.
+CREATE TABLE taste_profile_eras (
+    user_id    uuid             NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    era        text             NOT NULL,
+    weight     double precision NOT NULL,
+    updated_at timestamptz      NOT NULL DEFAULT now(),
+    PRIMARY KEY (user_id, era)
+);
+
+CREATE INDEX taste_profile_eras_user_weight_idx
+    ON taste_profile_eras (user_id, weight DESC);
+
+-- Era-facet build knob, tunable in the lab (mirrors enriched_tag_scale,
+-- #1520): scales how strongly a decade-play imprints on the profile.
+-- DEFAULT 0.5 backfills the existing singleton and matches
+-- taste.DefaultConfig().EraScale; 0 disables the era facet entirely.
+ALTER TABLE taste_tuning
+    ADD COLUMN era_scale double precision NOT NULL DEFAULT 0.5;