feat(taste): time-of-day / weekday context conditioning — #1531

Milestone #160 Opt 3 (temporal half). A new additive scoring term that
boosts a candidate when its artist's play history concentrates in the
CURRENT daypart × weekday-type cell, in the user's local timezone.

- Migration 0046: recommendation_weight_profiles.context_time_weight
  (per-profile scoring weight, DEFAULT 1.0).
- Query ListArtistContextPlayCountsForUser: per-artist completed-play
  counts split by the current cell (daypart night[22,5)/morning[5,12)/
  afternoon[12,17)/evening[17,22) × weekday-vs-weekend) via
  started_at AT TIME ZONE users.timezone; 365-day window, skips excluded.
- internal/recommendation/context.go: LoadContextAffinity computes each
  artist's shrunk cell-share minus the user's baseline share, clamped to
  [-1,1]; sparse artists shrink toward baseline (pseudo-count 5), unknown
  artists → 0 (cold-start neutral).
- Score() gains context_affinity_score · ContextTimeWeight; both
  candidate loaders set it per candidate.
- Tuning lab: ContextTimeWeight threaded through recsettings + admin API
  + web card ("Time-of-day weight" row) + Go/web tests. Shipped 1.0 both
  profiles (uniform start, re-bakeable).

Device-class axis deferred to #1551 (needs a client_id → device-class
mapping that doesn't exist yet).

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

internal/db/dbq/models.go

@@ -446,16 +446,17 @@ type RecommendationTuningAudit struct {
 }
 
 type RecommendationWeightProfile struct {
-	Profile          string
-	BaseWeight       float64
-	LikeBoost        float64
-	RecencyWeight    float64
-	SkipPenalty      float64
-	JitterMagnitude  float64
-	ContextWeight    float64
-	SimilarityWeight float64
-	TasteWeight      float64
-	UpdatedAt        pgtype.Timestamptz
+	Profile           string
+	BaseWeight        float64
+	LikeBoost         float64
+	RecencyWeight     float64
+	SkipPenalty       float64
+	JitterMagnitude   float64
+	ContextWeight     float64
+	SimilarityWeight  float64
+	TasteWeight       float64
+	UpdatedAt         pgtype.Timestamptz
+	ContextTimeWeight float64
 }
 
 type RegistrationSetting struct {