feat(recommendation): For You exploration attribution — taste vs fresh picks

Milestone #127 step 2 (#1249). For You deliberately blends two
populations — a head of top-scored taste picks and a tail sampled from
deeper ranking (the freshness injection) — but the metrics judged it as
one blob, so its skip rate couldn't distinguish "the taste engine is
missing" from "the freshness tax is too high". That number decides the
exploration share before we tune it.

- Migration 0038: nullable pick_kind ('taste'|'fresh') on both
  playlist_tracks (stamped at snapshot build) and play_events (frozen at
  play-ingestion — the snapshot rebuilds daily, so attribution cannot be
  reconstructed at read time).
- Builder: pickHeadAndTail marks head=taste / tail=fresh; the small-pool
  fallback is all taste (top-N-by-score IS the taste mechanism). Other
  variants persist NULL.
- Ingestion: for_you plays (live + offline replay) look the track up in
  the user's current snapshot; not found → unattributed, never guessed.
- Metrics: For You's row gains a breakdown (taste / fresh / earlier
  unattributed plays), parent row stays the sum; web card renders the
  sub-rows indented with the same baseline deltas + low-data dimming.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TsF3cNoKrqCYsU78cXC8U6

internal/db/dbq/events.sql.go

@@ -31,6 +31,33 @@ func (q *Queries) GetCurrentSessionVectorForUser(ctx context.Context, userID pgt
 	return session_vector_at_play, err
 }
 
+const getForYouPickKindForTrack = `-- name: GetForYouPickKindForTrack :one
+SELECT pt.pick_kind
+FROM playlist_tracks pt
+JOIN playlists p ON p.id = pt.playlist_id
+WHERE p.user_id = $1
+  AND p.system_variant = 'for_you'
+  AND pt.track_id = $2
+LIMIT 1
+`
+
+type GetForYouPickKindForTrackParams struct {
+	UserID  pgtype.UUID
+	TrackID pgtype.UUID
+}
+
+// Looks a track up in the user's CURRENT For You snapshot and returns
+// its pick_kind ('taste'/'fresh'). Used at play-ingestion time to
+// freeze exploration attribution onto the play_event — the snapshot is
+// rebuilt daily, so attribution can't be reconstructed at read time
+// (#1249). No row = track not in today's snapshot (caller stores NULL).
+func (q *Queries) GetForYouPickKindForTrack(ctx context.Context, arg GetForYouPickKindForTrackParams) (*string, error) {
+	row := q.db.QueryRow(ctx, getForYouPickKindForTrack, arg.UserID, arg.TrackID)
+	var pick_kind *string
+	err := row.Scan(&pick_kind)
+	return pick_kind, err
+}
+
 const getMostRecentPlaySessionForUser = `-- name: GetMostRecentPlaySessionForUser :one
 SELECT id, user_id, started_at, ended_at, last_event_at, track_count, client_id FROM play_sessions
 WHERE user_id = $1
@@ -54,7 +81,7 @@ func (q *Queries) GetMostRecentPlaySessionForUser(ctx context.Context, userID pg
 }
 
 const getOpenPlayEventForUser = `-- name: GetOpenPlayEventForUser :one
-SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source FROM play_events
+SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind FROM play_events
 WHERE user_id = $1 AND ended_at IS NULL
 ORDER BY started_at DESC
 LIMIT 1
@@ -79,12 +106,13 @@ func (q *Queries) GetOpenPlayEventForUser(ctx context.Context, userID pgtype.UUI
 		&i.SessionVectorAtPlay,
 		&i.ScrobbledAt,
 		&i.Source,
+		&i.PickKind,
 	)
 	return i, err
 }
 
 const getPlayEventByID = `-- name: GetPlayEventByID :one
-SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source FROM play_events WHERE id = $1
+SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind FROM play_events WHERE id = $1
 `
 
 func (q *Queries) GetPlayEventByID(ctx context.Context, id pgtype.UUID) (PlayEvent, error) {
@@ -104,15 +132,16 @@ func (q *Queries) GetPlayEventByID(ctx context.Context, id pgtype.UUID) (PlayEve
 		&i.SessionVectorAtPlay,
 		&i.ScrobbledAt,
 		&i.Source,
+		&i.PickKind,
 	)
 	return i, err
 }
 
 const insertPlayEvent = `-- name: InsertPlayEvent :one
 INSERT INTO play_events (
-    user_id, track_id, session_id, started_at, client_id, source
-) VALUES ($1, $2, $3, $4, $5, $6)
-RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source
+    user_id, track_id, session_id, started_at, client_id, source, pick_kind
+) VALUES ($1, $2, $3, $4, $5, $6, $7::text)
+RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind
 `
 
 type InsertPlayEventParams struct {
@@ -122,8 +151,11 @@ type InsertPlayEventParams struct {
 	StartedAt pgtype.Timestamptz
 	ClientID  *string
 	Source    *string
+	PickKind  *string
 }
 
+// pick_kind is non-NULL only for source='for_you' plays whose track was
+// found in the user's live For You snapshot at ingestion time (#1249).
 func (q *Queries) InsertPlayEvent(ctx context.Context, arg InsertPlayEventParams) (PlayEvent, error) {
 	row := q.db.QueryRow(ctx, insertPlayEvent,
 		arg.UserID,
@@ -132,6 +164,7 @@ func (q *Queries) InsertPlayEvent(ctx context.Context, arg InsertPlayEventParams
 		arg.StartedAt,
 		arg.ClientID,
 		arg.Source,
+		arg.PickKind,
 	)
 	var i PlayEvent
 	err := row.Scan(
@@ -148,6 +181,7 @@ func (q *Queries) InsertPlayEvent(ctx context.Context, arg InsertPlayEventParams
 		&i.SessionVectorAtPlay,
 		&i.ScrobbledAt,
 		&i.Source,
+		&i.PickKind,
 	)
 	return i, err
 }
@@ -291,7 +325,7 @@ SET ended_at = $2,
     completion_ratio = $4,
     was_skipped = $5
 WHERE id = $1
-RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source
+RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind
 `
 
 type UpdatePlayEventEndedParams struct {
@@ -328,6 +362,7 @@ func (q *Queries) UpdatePlayEventEnded(ctx context.Context, arg UpdatePlayEventE
 		&i.SessionVectorAtPlay,
 		&i.ScrobbledAt,
 		&i.Source,
+		&i.PickKind,
 	)
 	return i, err
 }