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
 }