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/playlists/discover_test.go

@@ -161,3 +161,27 @@ func TestInterleaveBuckets_DedupsAcrossBuckets(t *testing.T) {
 		}
 	}
 }
+
+func TestInterleaveBuckets_PreservesBucketPickKind(t *testing.T) {
+	// Bucket provenance (#1270) is stamped on discoverTrack before the
+	// interleave and must survive it — including dedup, where a track in
+	// two buckets keeps the stamp of the bucket it was taken from.
+	dormant := []discoverTrack{
+		{ID: uuidN(1), PickKind: pickKindDormant},
+		{ID: uuidN(2), PickKind: pickKindDormant},
+	}
+	random := []discoverTrack{
+		{ID: uuidN(1), PickKind: pickKindRandom}, // shared with dormant
+		{ID: uuidN(3), PickKind: pickKindRandom},
+	}
+	got := interleaveBuckets(dormant, nil, random)
+	if len(got) != 3 {
+		t.Fatalf("len = %d, want 3", len(got))
+	}
+	want := map[byte]string{1: pickKindDormant, 2: pickKindDormant, 3: pickKindRandom}
+	for _, tr := range got {
+		if w := want[tr.ID.Bytes[15]]; tr.PickKind != w {
+			t.Errorf("track %d pick_kind = %q, want %q", tr.ID.Bytes[15], tr.PickKind, w)
+		}
+	}
+}