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/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