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/recommendation/candidates.go

@@ -41,6 +41,11 @@ func LoadCandidates(
 		return nil, err
 	}
 
+	affinity, err := LoadContextAffinity(ctx, q, userID)
+	if err != nil {
+		return nil, err
+	}
+
 	out := make([]Candidate, 0, len(rows))
 	for _, r := range rows {
 		var lpt *time.Time
@@ -58,6 +63,7 @@ func LoadCandidates(
 				SkipCount:            int(r.SkipCount),
 				ContextualMatchScore: ctxScore,
 				TasteMatchScore:      profile.Match(r.Track.ArtistID, r.Track.Genre, r.ReleaseDate),
+				ContextAffinityScore: affinity.Affinity(r.Track.ArtistID),
 			},
 		})
 	}
@@ -134,6 +140,11 @@ func LoadCandidatesFromSimilarity(
 		return nil, err
 	}
 
+	affinity, err := LoadContextAffinity(ctx, q, userID)
+	if err != nil {
+		return nil, err
+	}
+
 	out := make([]Candidate, 0, len(rows))
 	for _, r := range rows {
 		var lpt *time.Time
@@ -159,6 +170,7 @@ func LoadCandidatesFromSimilarity(
 				ContextualMatchScore: ctxScore,
 				SimilarityScore:      simScore,
 				TasteMatchScore:      profile.Match(r.Track.ArtistID, r.Track.Genre, r.ReleaseDate),
+				ContextAffinityScore: affinity.Affinity(r.Track.ArtistID),
 			},
 		})
 	}