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/api/me_recommendation_metrics_test.go

@@ -168,6 +168,48 @@ func TestBucketMetricsResponse_ForYouBreakdown(t *testing.T) {
 	}
 }
 
+func TestBucketMetricsResponse_DiscoverBucketBreakdown(t *testing.T) {
+	// Provenance is standard (#1270): Discover's bucket stamps surface as
+	// a breakdown exactly like For You's taste/fresh — this is what makes
+	// the 40/30/30 allocation judgeable instead of a guess.
+	src := func(s string) *string { return &s }
+	kind := func(s string) *string { return &s }
+	rows := []dbq.RecommendationSourceMetricsForUserRow{
+		{Source: src("discover"), PickKind: kind("dormant"), Plays: 8, Skips: 2,
+			CompletionN: 8, AvgCompletion: 0.8},
+		{Source: src("discover"), PickKind: kind("cross_user"), Plays: 6, Skips: 3,
+			CompletionN: 6, AvgCompletion: 0.6},
+		{Source: src("discover"), PickKind: kind("random"), Plays: 4, Skips: 3,
+			CompletionN: 4, AvgCompletion: 0.4},
+		// NULL pick_kind = plays that predate bucket stamping.
+		{Source: src("discover"), Plays: 2, Skips: 0, CompletionN: 2, AvgCompletion: 0.9},
+	}
+	resp := bucketMetricsResponse(recMetricsDefaultDays, rows)
+
+	d := findSurface(resp, "discover")
+	if d == nil {
+		t.Fatal("discover family missing")
+	}
+	if d.Plays != 20 || d.Skips != 8 {
+		t.Errorf("discover plays/skips = %d/%d, want 20/8", d.Plays, d.Skips)
+	}
+	wantKeys := []string{
+		"discover_dormant", "discover_cross_user", "discover_random",
+		"discover_unattributed",
+	}
+	if len(d.Breakdown) != len(wantKeys) {
+		t.Fatalf("breakdown rows = %d, want %d", len(d.Breakdown), len(wantKeys))
+	}
+	for i, k := range wantKeys {
+		if d.Breakdown[i].Key != k {
+			t.Errorf("breakdown[%d].key = %s, want %s", i, d.Breakdown[i].Key, k)
+		}
+	}
+	if b := d.Breakdown[0]; b.Label != "Dormant artists" || b.Plays != 8 {
+		t.Errorf("dormant row = %+v, want label=Dormant artists plays=8", b)
+	}
+}
+
 func TestRecommendationMetrics_BucketsWithBaseline(t *testing.T) {
 	if os.Getenv("MINSTREL_TEST_DATABASE_URL") == "" {
 		t.Skip("MINSTREL_TEST_DATABASE_URL not set")