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/playlists/system.go

@@ -130,17 +130,18 @@ type rankedCandidate struct {
 // 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.
+// (#1270). Values mirror the CHECK in migration 0041; taste_unheard is
+// Discover's taste-targeted-novelty arm (#1488).
 const (
-	pickKindTaste     = "taste"
-	pickKindFresh     = "fresh"
-	pickKindDormant   = "dormant"
-	pickKindCrossUser = "cross_user"
-	pickKindRandom    = "random"
-	pickKindTier1     = "tier1"
-	pickKindTier2     = "tier2"
-	pickKindTier3     = "tier3"
+	pickKindTaste        = "taste"
+	pickKindFresh        = "fresh"
+	pickKindDormant      = "dormant"
+	pickKindTasteUnheard = "taste_unheard"
+	pickKindCrossUser    = "cross_user"
+	pickKindRandom       = "random"
+	pickKindTier1        = "tier1"
+	pickKindTier2        = "tier2"
+	pickKindTier3        = "tier3"
 )
 
 // pickKindForSeedTier maps PickSeedArtists' seed-pool fallback tier