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/db/queries/events.sql

@@ -24,25 +24,30 @@ ORDER BY started_at DESC
 LIMIT 1;
 
 -- name: InsertPlayEvent :one
--- pick_kind is non-NULL only for source='for_you' plays whose track was
--- found in the user's live For You snapshot at ingestion time (#1249).
+-- pick_kind is non-NULL only for system-playlist plays whose track was
+-- found (with a stamped kind) in the user's live snapshot for that
+-- variant at ingestion time (#1249, generalized in #1270).
 INSERT INTO play_events (
     user_id, track_id, session_id, started_at, client_id, source, pick_kind
 ) VALUES ($1, $2, $3, $4, $5, $6, sqlc.narg(pick_kind)::text)
 RETURNING *;
 
--- name: GetForYouPickKindForTrack :one
--- Looks a track up in the user's CURRENT For You snapshot and returns
--- its pick_kind ('taste'/'fresh'). Used at play-ingestion time to
--- freeze exploration attribution onto the play_event — the snapshot is
--- rebuilt daily, so attribution can't be reconstructed at read time
--- (#1249). No row = track not in today's snapshot (caller stores NULL).
+-- name: GetSystemPickKindForTrack :one
+-- Looks a track up in the user's CURRENT snapshot of the given system
+-- variant and returns its pick_kind. Used at play-ingestion time to
+-- freeze provenance onto the play_event — snapshots rebuild daily, so
+-- attribution can't be reconstructed at read time (#1249/#1270). No
+-- row = track not in today's snapshot (caller stores NULL); a row with
+-- NULL pick_kind = the variant doesn't stamp (yet). songs_like_artist
+-- is non-singleton (up to 3 mixes/user); a track in two of them takes
+-- whichever LIMIT 1 hits — acceptable, tier stamps there describe the
+-- same eligibility ladder.
 SELECT pt.pick_kind
 FROM playlist_tracks pt
 JOIN playlists p ON p.id = pt.playlist_id
 WHERE p.user_id = $1
-  AND p.system_variant = 'for_you'
-  AND pt.track_id = $2
+  AND p.system_variant = $2
+  AND pt.track_id = $3
 LIMIT 1;
 
 -- name: UpdatePlayEventEnded :one