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_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,