feat(metrics): provenance as standard — pick_kind for all system mixes

The #1249 mechanism (stamp WHY a track is in the snapshot at build
time, freeze it onto the play at ingestion, break it down in metrics)
generalizes from a For You one-off to the standard for every system
mix (#1270):

- Migration 0039 widens both pick_kind CHECKs (drop + re-add in the
  same change) to taste/fresh + Discover's dormant/cross_user/random
  + tier1-3 for the rule-#131 eligibility ladders.
- GetForYouPickKindForTrack becomes GetSystemPickKindForTrack
  (user, variant, track); ingestion stamps any systemPlaylistSources
  play from its own variant's live snapshot, live + offline paths.
- Discover stamps its candidate bucket on discoverTrack before the
  interleave, making the 40/30/30 allocation measurable; dedup keeps
  the taking bucket's stamp.
- Metrics replace the for_you special-case with one pick-kind
  vocabulary — any family with attributed plays gets a breakdown,
  future stamping mixes need no metrics change.
- Web: breakdown sub-rows are now toggled per surface (collapsed by
  default) so eight stamping mixes don't swamp the card.

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

internal/db/dbq/events.sql.go

@@ -31,33 +31,6 @@ 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
@@ -137,6 +110,38 @@ func (q *Queries) GetPlayEventByID(ctx context.Context, id pgtype.UUID) (PlayEve
 	return i, err
 }
 
+const getSystemPickKindForTrack = `-- name: GetSystemPickKindForTrack :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 = $2
+  AND pt.track_id = $3
+LIMIT 1
+`
+
+type GetSystemPickKindForTrackParams struct {
+	UserID        pgtype.UUID
+	SystemVariant *string
+	TrackID       pgtype.UUID
+}
+
+// Looks a track up in the user's CURRENT snapshot of the given system
+// variant and returns its pick_kind. Used at play-ingestion time to
+// freeze provenance onto the play_event — snapshots rebuild daily, so
+// attribution can't be reconstructed at read time (#1249/#1270). No
+// row = track not in today's snapshot (caller stores NULL); a row with
+// NULL pick_kind = the variant doesn't stamp (yet). songs_like_artist
+// is non-singleton (up to 3 mixes/user); a track in two of them takes
+// whichever LIMIT 1 hits — acceptable, tier stamps there describe the
+// same eligibility ladder.
+func (q *Queries) GetSystemPickKindForTrack(ctx context.Context, arg GetSystemPickKindForTrackParams) (*string, error) {
+	row := q.db.QueryRow(ctx, getSystemPickKindForTrack, arg.UserID, arg.SystemVariant, arg.TrackID)
+	var pick_kind *string
+	err := row.Scan(&pick_kind)
+	return pick_kind, err
+}
+
 const insertPlayEvent = `-- name: InsertPlayEvent :one
 INSERT INTO play_events (
     user_id, track_id, session_id, started_at, client_id, source, pick_kind
@@ -154,8 +159,9 @@ type InsertPlayEventParams struct {
 	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).
+// pick_kind is non-NULL only for system-playlist plays whose track was
+// found (with a stamped kind) in the user's live snapshot for that
+// variant at ingestion time (#1249, generalized in #1270).
 func (q *Queries) InsertPlayEvent(ctx context.Context, arg InsertPlayEventParams) (PlayEvent, error) {
 	row := q.db.QueryRow(ctx, insertPlayEvent,
 		arg.UserID,