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/recommendation/taste_test.go

@@ -17,6 +17,14 @@ func uuidN(n byte) pgtype.UUID {
 
 func strPtr(s string) *string { return &s }
 
+// noDate is an absent release date — the era term contributes 0.
+func noDate() pgtype.Date { return pgtype.Date{} }
+
+// dateInYear builds a valid release date in the given year (era = its decade).
+func dateInYear(year int) pgtype.Date {
+	return pgtype.Date{Time: time.Date(year, 1, 1, 0, 0, 0, 0, time.UTC), Valid: true}
+}
+
 func TestTasteProfile_Match(t *testing.T) {
 	loved := uuidN(1)
 	disliked := uuidN(2)
@@ -26,32 +34,55 @@ func TestTasteProfile_Match(t *testing.T) {
 		tags:    map[string]float64{"Jazz": 6.0, "Noise": -6.0},
 	}
 
-	if m := p.Match(loved, strPtr("Jazz")); m <= 0.5 {
+	if m := p.Match(loved, strPtr("Jazz"), noDate()); m <= 0.5 {
 		t.Errorf("loved artist + loved tag = %.3f, want strongly positive", m)
 	}
-	if m := p.Match(disliked, strPtr("Noise")); m >= -0.5 {
+	if m := p.Match(disliked, strPtr("Noise"), noDate()); m >= -0.5 {
 		t.Errorf("disliked artist + disliked tag = %.3f, want strongly negative", m)
 	}
-	if m := p.Match(unknown, nil); m != 0 {
+	if m := p.Match(unknown, nil, noDate()); m != 0 {
 		t.Errorf("unknown artist, no genre = %.3f, want 0", m)
 	}
 	// Artist dominates (0.7 share): loved artist with an unknown tag is still
 	// clearly positive.
-	if m := p.Match(loved, strPtr("Unheard")); m <= 0 {
+	if m := p.Match(loved, strPtr("Unheard"), noDate()); m <= 0 {
 		t.Errorf("loved artist + unknown tag = %.3f, want positive", m)
 	}
 	// Output stays within [-1, 1] even with saturated inputs.
 	for _, a := range []pgtype.UUID{loved, disliked, unknown} {
-		m := p.Match(a, strPtr("Jazz"))
+		m := p.Match(a, strPtr("Jazz"), dateInYear(1994))
 		if m < -1 || m > 1 {
 			t.Errorf("Match out of [-1,1]: %.3f", m)
 		}
 	}
 }
 
+// TestTasteProfile_EraTerm verifies the decade facet nudges the match: with
+// artist + genre held neutral, a loved era lifts the score and a disliked era
+// lowers it, while an undated track is unaffected.
+func TestTasteProfile_EraTerm(t *testing.T) {
+	art := uuidN(1)
+	p := TasteProfile{
+		artists: map[pgtype.UUID]float64{},
+		tags:    map[string]float64{},
+		eras:    map[string]float64{"1990s": 8.0, "1980s": -8.0},
+	}
+	loved := p.Match(art, nil, dateInYear(1994))
+	if loved <= 0 {
+		t.Errorf("loved era (1990s) = %.3f, want positive", loved)
+	}
+	disliked := p.Match(art, nil, dateInYear(1987))
+	if disliked >= 0 {
+		t.Errorf("disliked era (1980s) = %.3f, want negative", disliked)
+	}
+	if m := p.Match(art, nil, noDate()); m != 0 {
+		t.Errorf("undated track = %.3f, want 0 (no era contribution)", m)
+	}
+}
+
 func TestTasteProfile_EmptyIsNeutral(t *testing.T) {
 	var p TasteProfile // zero value: nil maps
-	if m := p.Match(uuidN(1), strPtr("Jazz")); m != 0 {
+	if m := p.Match(uuidN(1), strPtr("Jazz"), dateInYear(1994)); m != 0 {
 		t.Errorf("empty profile Match = %.3f, want 0 (cold start neutral)", m)
 	}
 }
@@ -100,10 +131,10 @@ func TestLoadTasteProfile_RoundTrip(t *testing.T) {
 	if err != nil {
 		t.Fatalf("load: %v", err)
 	}
-	if m := p.Match(art.ID, strPtr("Jazz")); m <= 0.5 {
+	if m := p.Match(art.ID, strPtr("Jazz"), noDate()); m <= 0.5 {
 		t.Errorf("round-trip Match = %.3f, want strongly positive", m)
 	}
-	if m := p.Match(uuidN(9), nil); m != 0 {
+	if m := p.Match(uuidN(9), nil, noDate()); m != 0 {
 		t.Errorf("absent artist Match = %.3f, want 0", m)
 	}
 }