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/context_test.go

@@ -0,0 +1,57 @@
+package recommendation
+
+import (
+	"testing"
+	"time"
+)
+
+func TestContextAffinity(t *testing.T) {
+	const baseline = 0.4 // 40% of the user's plays fall in the current cell
+	const k = contextAffinityShrinkage
+
+	// Heavy history, over-represented in the current cell → positive.
+	if a := contextAffinity(80, 100, baseline, k); a <= 0 {
+		t.Errorf("over-represented artist affinity = %.3f, want positive", a)
+	}
+	// Heavy history, under-represented → negative.
+	if a := contextAffinity(10, 100, baseline, k); a >= 0 {
+		t.Errorf("under-represented artist affinity = %.3f, want negative", a)
+	}
+	// A sparse artist (1/1) shrinks toward the baseline, so its affinity is
+	// smaller than a heavily-played artist with the same raw cell-share.
+	sparse := contextAffinity(1, 1, baseline, k)
+	heavy := contextAffinity(100, 100, baseline, k)
+	if sparse >= heavy {
+		t.Errorf("sparse (%.3f) should shrink below heavy (%.3f)", sparse, heavy)
+	}
+	// No plays → neutral.
+	if a := contextAffinity(0, 0, baseline, k); a != 0 {
+		t.Errorf("no plays affinity = %.3f, want 0", a)
+	}
+	// Result stays within [-1, 1].
+	for _, tc := range [][2]float64{{100, 100}, {0, 100}, {50, 50}} {
+		a := contextAffinity(tc[0], tc[1], baseline, k)
+		if a < -1 || a > 1 {
+			t.Errorf("affinity out of [-1,1]: %.3f", a)
+		}
+	}
+}
+
+func TestScore_ContextTermAddsAndSubtracts(t *testing.T) {
+	now := time.Now()
+	zeroJitter := func() float64 { return 0.5 } // (0.5*2-1)=0 with any magnitude
+	w := ScoringWeights{ContextTimeWeight: 2.0} // all other weights 0
+
+	pos := Score(ScoringInputs{ContextAffinityScore: 1.0}, w, now, zeroJitter)
+	if !almostEq(pos, 2.0) {
+		t.Errorf("positive context affinity: Score = %.3f, want 2.0", pos)
+	}
+	neg := Score(ScoringInputs{ContextAffinityScore: -1.0}, w, now, zeroJitter)
+	if !almostEq(neg, -2.0) {
+		t.Errorf("negative context affinity: Score = %.3f, want -2.0 (demotes)", neg)
+	}
+	off := Score(ScoringInputs{ContextAffinityScore: 1.0}, ScoringWeights{}, now, zeroJitter)
+	if !almostEq(off, 0.0) {
+		t.Errorf("ContextTimeWeight 0: Score = %.3f, want 0 (no effect)", off)
+	}
+}