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.go

@@ -0,0 +1,70 @@
+package recommendation
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// contextAffinityShrinkage is the pseudo-count that pulls a low-play artist's
+// cell-share toward the user's baseline, so an artist with one or two plays
+// can't swing its affinity to ±1 on noise. At k plays the estimate sits
+// halfway between the raw cell-share and the baseline.
+const contextAffinityShrinkage = 5.0
+
+// ContextAffinity is the read-side map of per-artist time-of-day/weekday
+// affinity for the CURRENT context (#1531): artist_id → score in [-1, +1].
+// Absent artists (no play history) score 0, so cold-start candidates stay
+// neutral. The zero value is a valid all-neutral affinity.
+type ContextAffinity struct {
+	byArtist map[pgtype.UUID]float64
+}
+
+// Affinity returns the artist's current-context affinity, or 0 if unknown.
+func (c ContextAffinity) Affinity(artistID pgtype.UUID) float64 {
+	return c.byArtist[artistID]
+}
+
+// LoadContextAffinity computes each artist's affinity for the user's CURRENT
+// daypart × weekday cell. For every artist with completed plays in the window
+// it compares the share of that artist's plays that fall in the current cell
+// against the user's overall baseline share, shrinking sparse artists toward
+// the baseline. Returns an empty (all-neutral) affinity when the user has no
+// plays.
+func LoadContextAffinity(
+	ctx context.Context, q *dbq.Queries, userID pgtype.UUID,
+) (ContextAffinity, error) {
+	rows, err := q.ListArtistContextPlayCountsForUser(ctx, userID)
+	if err != nil {
+		return ContextAffinity{}, err
+	}
+	var totalPlays, cellPlays int64
+	for _, r := range rows {
+		totalPlays += r.TotalPlays
+		cellPlays += r.CellPlays
+	}
+	out := ContextAffinity{byArtist: make(map[pgtype.UUID]float64, len(rows))}
+	if totalPlays == 0 {
+		return out, nil
+	}
+	baseline := float64(cellPlays) / float64(totalPlays)
+	for _, r := range rows {
+		out.byArtist[r.ArtistID] = contextAffinity(
+			float64(r.CellPlays), float64(r.TotalPlays), baseline, contextAffinityShrinkage)
+	}
+	return out, nil
+}
+
+// contextAffinity returns an artist's shrunk cell-share minus the user's
+// baseline share, clamped to [-1, 1]. The shrinkage pseudo-count k pulls
+// low-play artists toward the baseline (→ 0 affinity) so noise can't dominate;
+// a heavily-played artist keeps close to its raw over/under-representation.
+func contextAffinity(cellPlays, totalPlays, baseline, k float64) float64 {
+	if totalPlays == 0 {
+		return 0
+	}
+	shrunk := (cellPlays + baseline*k) / (totalPlays + k)
+	return clampUnit(shrunk - baseline)
+}