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

web/src/lib/api/metrics.ts

@@ -12,8 +12,10 @@ export type SurfaceMetric = {
   skip_rate: number;
   avg_completion: number;
   low_confidence: boolean;
-  // Only For You carries a breakdown (#1249): taste picks vs fresh picks
-  // (the deliberate freshness injection), plus pre-attribution plays.
+  // Present when the surface's builder stamps pick-kind provenance and
+  // the window holds attributed plays (#1249, generalized #1270): For
+  // You's taste/fresh split, Discover's candidate buckets, the tiered
+  // mixes' tier1-3 — plus pre-attribution plays.
   breakdown?: SurfaceMetric[];
 };