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.go

@@ -20,11 +20,15 @@ const (
 
 // discoverTrack is the common shape used by the bucket allocator. The
 // three sqlc-generated row types collapse into this internal struct so
-// downstream functions don't need to be generic over them.
+// downstream functions don't need to be generic over them. PickKind is
+// the originating bucket (dormant/cross_user/random, #1270), stamped in
+// the row adapters so it survives the interleave — bucket identity is
+// what makes the 40/30/30 allocation measurable in the metrics.
 type discoverTrack struct {
 	ID       pgtype.UUID
 	AlbumID  pgtype.UUID
 	ArtistID pgtype.UUID
+	PickKind string
 }
 
 // buildDiscoverCandidates assembles the Discover playlist track list.
@@ -91,7 +95,7 @@ func buildDiscoverCandidates(ctx context.Context, q *dbq.Queries, logger *slog.L
 	// already gave us the ranking.
 	ranked := make([]rankedCandidate, 0, len(out))
 	for _, t := range out {
-		ranked = append(ranked, rankedCandidate{TrackID: t.ID})
+		ranked = append(ranked, rankedCandidate{TrackID: t.ID, PickKind: t.PickKind})
 	}
 	return ranked, nil
 }
@@ -99,7 +103,10 @@ func buildDiscoverCandidates(ctx context.Context, q *dbq.Queries, logger *slog.L
 func dormantRowsToTracks(rows []dbq.ListDormantArtistTracksForDiscoverRow) []discoverTrack {
 	out := make([]discoverTrack, len(rows))
 	for i, r := range rows {
-		out[i] = discoverTrack{ID: r.ID, AlbumID: r.AlbumID, ArtistID: r.ArtistID}
+		out[i] = discoverTrack{
+			ID: r.ID, AlbumID: r.AlbumID, ArtistID: r.ArtistID,
+			PickKind: pickKindDormant,
+		}
 	}
 	return out
 }
@@ -107,7 +114,10 @@ func dormantRowsToTracks(rows []dbq.ListDormantArtistTracksForDiscoverRow) []dis
 func crossUserRowsToTracks(rows []dbq.ListCrossUserLikedTracksForDiscoverRow) []discoverTrack {
 	out := make([]discoverTrack, len(rows))
 	for i, r := range rows {
-		out[i] = discoverTrack{ID: r.ID, AlbumID: r.AlbumID, ArtistID: r.ArtistID}
+		out[i] = discoverTrack{
+			ID: r.ID, AlbumID: r.AlbumID, ArtistID: r.ArtistID,
+			PickKind: pickKindCrossUser,
+		}
 	}
 	return out
 }
@@ -115,7 +125,10 @@ func crossUserRowsToTracks(rows []dbq.ListCrossUserLikedTracksForDiscoverRow) []
 func randomRowsToTracks(rows []dbq.ListRandomUnheardTracksForDiscoverRow) []discoverTrack {
 	out := make([]discoverTrack, len(rows))
 	for i, r := range rows {
-		out[i] = discoverTrack{ID: r.ID, AlbumID: r.AlbumID, ArtistID: r.ArtistID}
+		out[i] = discoverTrack{
+			ID: r.ID, AlbumID: r.AlbumID, ArtistID: r.ArtistID,
+			PickKind: pickKindRandom,
+		}
 	}
 	return out
 }

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)
+		}
+	}
+}

internal/playlists/system.go

@@ -127,20 +127,26 @@ func pickSeedArtistsForDay(pool []pgtype.UUID, userID pgtype.UUID, dateStr strin
 
 // rankedCandidate is a (track_id, score) pair used during in-memory
 // sorting before insert into playlist_tracks. T5 fills these from
-// recommendation.Candidate scores. PickKind is set only by For-You's
-// head/tail composition (#1249); empty persists as NULL.
+// recommendation.Candidate scores. PickKind is the track's provenance
+// within its mix — For-You's head/tail split (#1249) and Discover's
+// buckets (#1270); empty persists as NULL (variant doesn't stamp yet).
 type rankedCandidate struct {
 	TrackID  pgtype.UUID
 	Score    float64
 	PickKind string
 }
 
-// For-You pick kinds, persisted on playlist_tracks.pick_kind so plays
-// can attribute skips to "the taste engine missing" vs "the freshness
-// tax" (#1249). Values mirror the CHECK in migration 0038.
+// Pick kinds, persisted on playlist_tracks.pick_kind so plays can
+// attribute skips to the population that sourced the track — For You's
+// "taste engine missing" vs "freshness tax", Discover's three buckets
+// (#1270). Values mirror the CHECK in migration 0039; the tier1-3
+// values there land with the tiered-mix rebuilds.
 const (
-	pickKindTaste = "taste"
-	pickKindFresh = "fresh"
+	pickKindTaste     = "taste"
+	pickKindFresh     = "fresh"
+	pickKindDormant   = "dormant"
+	pickKindCrossUser = "cross_user"
+	pickKindRandom    = "random"
 )
 
 const systemMixLength = 25