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/recommendation.sql.go

@@ -11,6 +11,83 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
+const listArtistContextPlayCountsForUser = `-- name: ListArtistContextPlayCountsForUser :many
+WITH tz AS (
+    SELECT COALESCE(NULLIF(u.timezone, ''), 'UTC') AS zone
+    FROM users u WHERE u.id = $1
+),
+now_cell AS (
+    SELECT
+        CASE
+            WHEN EXTRACT(hour FROM now() AT TIME ZONE tz.zone) < 5  THEN 3
+            WHEN EXTRACT(hour FROM now() AT TIME ZONE tz.zone) < 12 THEN 0
+            WHEN EXTRACT(hour FROM now() AT TIME ZONE tz.zone) < 17 THEN 1
+            WHEN EXTRACT(hour FROM now() AT TIME ZONE tz.zone) < 22 THEN 2
+            ELSE 3
+        END AS daypart,
+        (EXTRACT(isodow FROM now() AT TIME ZONE tz.zone) >= 6) AS is_weekend
+    FROM tz
+),
+plays AS (
+    SELECT t.artist_id,
+           CASE
+               WHEN EXTRACT(hour FROM pe.started_at AT TIME ZONE tz.zone) < 5  THEN 3
+               WHEN EXTRACT(hour FROM pe.started_at AT TIME ZONE tz.zone) < 12 THEN 0
+               WHEN EXTRACT(hour FROM pe.started_at AT TIME ZONE tz.zone) < 17 THEN 1
+               WHEN EXTRACT(hour FROM pe.started_at AT TIME ZONE tz.zone) < 22 THEN 2
+               ELSE 3
+           END AS daypart,
+           (EXTRACT(isodow FROM pe.started_at AT TIME ZONE tz.zone) >= 6) AS is_weekend
+    FROM play_events pe
+    JOIN tracks t ON t.id = pe.track_id
+    CROSS JOIN tz
+    WHERE pe.user_id = $1
+      AND pe.was_skipped = false
+      AND pe.started_at > now() - interval '365 days'
+)
+SELECT p.artist_id,
+       count(*) AS total_plays,
+       count(*) FILTER (
+           WHERE p.daypart = (SELECT daypart FROM now_cell)
+             AND p.is_weekend = (SELECT is_weekend FROM now_cell)
+       ) AS cell_plays
+FROM plays p
+GROUP BY p.artist_id
+`
+
+type ListArtistContextPlayCountsForUserRow struct {
+	ArtistID   pgtype.UUID
+	TotalPlays int64
+	CellPlays  int64
+}
+
+// Per-artist completed-play counts split by whether each play falls in the
+// CURRENT daypart × weekday-type cell, in the user's local timezone (#1531).
+// Feeds the context-affinity scoring term: an artist whose plays concentrate
+// in the current cell (vs the user's overall baseline, computed Go-side from
+// these rows) gets boosted right now. Skips excluded; a 365-day window bounds
+// cost. Daypart buckets: night [22,5) morning [5,12) afternoon [12,17)
+// evening [17,22). Weekend = ISO days 6–7 (Sat/Sun).
+func (q *Queries) ListArtistContextPlayCountsForUser(ctx context.Context, id pgtype.UUID) ([]ListArtistContextPlayCountsForUserRow, error) {
+	rows, err := q.db.Query(ctx, listArtistContextPlayCountsForUser, id)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ListArtistContextPlayCountsForUserRow
+	for rows.Next() {
+		var i ListArtistContextPlayCountsForUserRow
+		if err := rows.Scan(&i.ArtistID, &i.TotalPlays, &i.CellPlays); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listLastPlayedArtistsForUser = `-- name: ListLastPlayedArtistsForUser :many
 WITH user_plays AS (
     SELECT t.artist_id, max(pe.started_at) AS last_started