feat(taste): phase 2a — apply the taste profile via a TasteMatch scoring term (#796)

The profile built in phase 1 now changes what gets surfaced. Adds a TasteMatch
term to the weighted-shuffle score so candidates are re-ranked by their fit to
the user's learned taste (positive draws toward it; negative reflects passive
avoidance; 0 at cold start).

- recommendation/score.go: ScoringInputs.TasteMatchScore ([-1,+1]) +
  ScoringWeights.TasteWeight + the term in Score.
- recommendation/taste.go: LoadTasteProfile reads the taste_profile_* tables;
  TasteProfile.Match blends the candidate's artist weight (0.7) and avg genre-tag
  weight (0.3), each tanh-squashed by a fixed scale so one outlier artist can't
  compress the rest. Unknown artist/tags and empty profiles → 0 (neutral).
- candidates.go: both candidate loaders set TasteMatchScore per candidate, so
  every Score caller (system playlists incl. You-might-like, radio) becomes
  taste-aware automatically.
- weights: systemMixWeights.TasteWeight = 1.5 (daily mixes are the primary
  taste surface); config.RecommendationConfig gains taste_weight (default 1.0,
  lighter — radio is seed-directed) wired into the radio handler.
- tests: pure (Match curve incl. saturation/clamp/empty-neutral, Score term
  add+subtract) + DB round-trip (seed taste rows → Match positive). All green
  vs real Postgres; existing playlist/radio tests unaffected (empty profile →
  zero taste effect).

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

internal/config/config.go

@@ -98,6 +98,7 @@ type RecommendationConfig struct {
 	JitterMagnitude     float64 `yaml:"jitter_magnitude"`
 	ContextWeight       float64 `yaml:"context_weight"`
 	SimilarityWeight    float64 `yaml:"similarity_weight"`
+	TasteWeight         float64 `yaml:"taste_weight"`
 	RecentlyPlayedHours int     `yaml:"recently_played_hours"`
 	RadioSize           int     `yaml:"radio_size"`
 	RadioSizeMax        int     `yaml:"radio_size_max"`
@@ -119,13 +120,16 @@ func Default() Config {
 			SkipMaxDurationPlayedMs: 30000,
 		},
 		Recommendation: RecommendationConfig{
-			BaseWeight:          1.0,
-			LikeBoost:           2.0,
-			RecencyWeight:       1.0,
-			SkipPenalty:         1.0,
-			JitterMagnitude:     0.1,
-			ContextWeight:       2.0,
-			SimilarityWeight:    2.0,
+			BaseWeight:       1.0,
+			LikeBoost:        2.0,
+			RecencyWeight:    1.0,
+			SkipPenalty:      1.0,
+			JitterMagnitude:  0.1,
+			ContextWeight:    2.0,
+			SimilarityWeight: 2.0,
+			// Radio is seed-directed (the user picked a direction), so taste
+			// is a lighter nudge here than in the daily mixes (1.5).
+			TasteWeight:         1.0,
 			RecentlyPlayedHours: 1,
 			RadioSize:           50,
 			RadioSizeMax:        200,