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/playevents/writer.go

@@ -97,25 +97,26 @@ var systemPlaylistSources = map[string]bool{
 	"songs_like_artist": true,
 }
 
-// sourceForYou is the one source whose plays carry exploration
-// attribution (taste vs fresh pick, #1249).
-const sourceForYou = "for_you"
-
-// lookupForYouPickKind resolves a For-You play's pick_kind ('taste' or
-// 'fresh') from the user's live snapshot at ingestion time. Attribution
-// must be frozen now — the snapshot rebuilds daily, so it can't be
-// reconstructed at metrics-read time. Returns nil (unattributed) when
-// the track isn't in the current snapshot: plays predating the feature,
-// or an offline replay arriving after the track rotated out. Real DB
+// lookupSystemPickKind resolves a system-playlist play's pick_kind
+// (taste/fresh, Discover's bucket, or a rule-#131 tier) from the user's
+// live snapshot of that variant at ingestion time. Attribution must be
+// frozen now — snapshots rebuild daily, so it can't be reconstructed at
+// metrics-read time (#1249, generalized in #1270). Source strings
+// double as system_variant values (see systemPlaylistSources). Returns
+// nil (unattributed) when the track isn't in the current snapshot —
+// plays predating the feature, or an offline replay arriving after the
+// track rotated out — and when the variant doesn't stamp yet. Real DB
 // errors propagate and fail the caller's transaction.
-func lookupForYouPickKind(
+func lookupSystemPickKind(
 	ctx context.Context,
 	q *dbq.Queries,
 	userID, trackID pgtype.UUID,
+	variant string,
 ) (*string, error) {
-	kind, err := q.GetForYouPickKindForTrack(ctx, dbq.GetForYouPickKindForTrackParams{
-		UserID:  userID,
-		TrackID: trackID,
+	kind, err := q.GetSystemPickKindForTrack(ctx, dbq.GetSystemPickKindForTrackParams{
+		UserID:        userID,
+		SystemVariant: &variant,
+		TrackID:       trackID,
 	})
 	if errors.Is(err, pgx.ErrNoRows) {
 		return nil, nil
@@ -157,8 +158,8 @@ func (w *Writer) RecordPlayStartedWithSource(
 			sourcePtr = &source
 		}
 		var pickKind *string
-		if source == sourceForYou {
-			pickKind, err = lookupForYouPickKind(ctx, q, userID, trackID)
+		if systemPlaylistSources[source] {
+			pickKind, err = lookupSystemPickKind(ctx, q, userID, trackID, source)
 			if err != nil {
 				return err
 			}
@@ -433,8 +434,8 @@ func (w *Writer) RecordOfflinePlay(
 			sourcePtr = &source
 		}
 		var pickKind *string
-		if source == sourceForYou {
-			pickKind, err = lookupForYouPickKind(ctx, q, userID, trackID)
+		if systemPlaylistSources[source] {
+			pickKind, err = lookupSystemPickKind(ctx, q, userID, trackID, source)
 			if err != nil {
 				return err
 			}