feat(discover): taste-targeted novelty bucket + rebalance toward discovery

The 2-week metrics review found Discover beating the manual baseline on
skip rate — which for a discovery surface means it plays it safe. On a
single-user server it's effectively dormant + crude-random, and the
per-user taste profile (taste_profile_tags, #796) went unused (#1254 gap).

Add a fourth Discover candidate bucket, ListTasteUnheardTracksForDiscover:
unheard / non-liked / non-quarantined tracks ranked by summed taste-tag
weight over tracks.genre (split like the radio tag_overlap arm), md5
tiebreak. Its picks are stamped pick_kind = 'taste_unheard' (migration
0041 widens the CHECK on playlist_tracks + play_events, rule #36).

Rebalance the slot allocation 40/30/30 → taste_unheard 35 / dormant 30 /
cross_user 20 / random 15, and lead the interleave with taste_unheard so
a track shared with another bucket keeps the taste stamp and the
targeted-novelty arm stays measurable. Metrics label "Taste-matched" +
order entry added to the single server-side pickKindLabels map, so web
and Android surface the new breakdown row with no client change.

Cold start (empty taste_profile_tags) yields an empty taste bucket that
redistributes to the survivors, so Discover still fills.

Scribe #1488. Companion review outcomes: Songs-like starvation already
fixed (#1255); For You v2 ratified as-is (fresh-injection cost disproven).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/api/me_recommendation_metrics_test.go

@@ -171,7 +171,7 @@ 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.
+	// the bucket allocation judgeable instead of a guess.
 	src := func(s string) *string { return &s }
 	kind := func(s string) *string { return &s }
 	rows := []dbq.RecommendationSourceMetricsForUserRow{