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
 			}

internal/playevents/writer_test.go

@@ -489,14 +489,13 @@ func TestRecordPlayStarted_NoRotationWithoutSource(t *testing.T) {
 	}
 }
 
-// seedForYouSnapshot creates a system For You playlist containing the
-// fixture track with the given pick_kind, mimicking what the builder
-// persists (#1249).
-func seedForYouSnapshot(t *testing.T, f fixture, pickKind string) {
+// seedSystemSnapshot creates a system playlist of the given variant
+// containing the fixture track with the given pick_kind, mimicking
+// what the builder persists (#1249, generalized #1270).
+func seedSystemSnapshot(t *testing.T, f fixture, variant, pickKind string) {
 	t.Helper()
-	variant := "for_you"
 	pl, err := f.q.CreateSystemPlaylist(context.Background(), dbq.CreateSystemPlaylistParams{
-		UserID: f.user, Name: "For You", SystemVariant: &variant,
+		UserID: f.user, Name: variant, SystemVariant: &variant,
 	})
 	if err != nil {
 		t.Fatalf("CreateSystemPlaylist: %v", err)
@@ -510,7 +509,7 @@ func seedForYouSnapshot(t *testing.T, f fixture, pickKind string) {
 
 func TestRecordPlayStartedWithSource_StampsForYouPickKind(t *testing.T) {
 	f := newFixture(t, 200_000)
-	seedForYouSnapshot(t, f, "fresh")
+	seedSystemSnapshot(t, f, "for_you", "fresh")
 	res, err := f.w.RecordPlayStartedWithSource(
 		context.Background(), f.user, f.track, "c", "for_you", time.Now().UTC(),
 	)
@@ -545,11 +544,13 @@ func TestRecordPlayStartedWithSource_NoSnapshotMatch_PickKindNull(t *testing.T)
 	}
 }
 
-func TestRecordPlayStartedWithSource_NonForYouSource_PickKindNull(t *testing.T) {
-	// Even with a For You snapshot containing the track, a play from a
-	// different surface must not pick up For You attribution.
+func TestRecordPlayStartedWithSource_CrossVariant_PickKindNull(t *testing.T) {
+	// Attribution is variant-scoped: even with a For You snapshot
+	// containing the track, a play from a different surface must not
+	// pick up For You's stamp — it reads its OWN variant's snapshot,
+	// which here doesn't exist.
 	f := newFixture(t, 200_000)
-	seedForYouSnapshot(t, f, "taste")
+	seedSystemSnapshot(t, f, "for_you", "taste")
 	res, err := f.w.RecordPlayStartedWithSource(
 		context.Background(), f.user, f.track, "c", "discover", time.Now().UTC(),
 	)
@@ -561,7 +562,27 @@ func TestRecordPlayStartedWithSource_NonForYouSource_PickKindNull(t *testing.T)
 		t.Fatalf("get: %v", err)
 	}
 	if got.PickKind != nil {
-		t.Errorf("pick_kind = %q, want NULL for non-for_you source", *got.PickKind)
+		t.Errorf("pick_kind = %q, want NULL for a cross-variant play", *got.PickKind)
+	}
+}
+
+func TestRecordPlayStartedWithSource_StampsDiscoverBucket(t *testing.T) {
+	// Discover stamps its candidate bucket (#1270); a discover play whose
+	// track is in the live snapshot freezes that bucket onto the play.
+	f := newFixture(t, 200_000)
+	seedSystemSnapshot(t, f, "discover", "dormant")
+	res, err := f.w.RecordPlayStartedWithSource(
+		context.Background(), f.user, f.track, "c", "discover", time.Now().UTC(),
+	)
+	if err != nil {
+		t.Fatalf("RecordPlayStartedWithSource: %v", err)
+	}
+	got, err := f.q.GetPlayEventByID(context.Background(), res.PlayEventID)
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if got.PickKind == nil || *got.PickKind != "dormant" {
+		t.Errorf("pick_kind = %v, want dormant (frozen from live snapshot)", got.PickKind)
 	}
 }
 
@@ -569,7 +590,7 @@ func TestRecordOfflinePlay_StampsForYouPickKind(t *testing.T) {
 	// The offline replay path threads source the same way; a replayed
 	// for_you play whose track is still in the snapshot gets attributed.
 	f := newFixture(t, 200_000)
-	seedForYouSnapshot(t, f, "taste")
+	seedSystemSnapshot(t, f, "for_you", "taste")
 	at := time.Now().UTC().Add(-time.Hour)
 	if err := f.w.RecordOfflinePlay(
 		context.Background(), f.user, f.track, "c", "for_you", at, 180_000,