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/db/queries/discover.sql

@@ -102,3 +102,37 @@ SELECT t.id, t.album_id, t.artist_id
    )
  ORDER BY md5(t.id::text || $2::text)
  LIMIT 200;
+
+-- name: ListTasteUnheardTracksForDiscover :many
+-- Taste-targeted novelty: unheard tracks whose genres overlap the user's
+-- taste-profile tags (taste_profile_tags, #796), ranked by summed tag
+-- weight — "new to you, but your vibe" rather than the crude random arm.
+-- Genres live inline on tracks.genre as a delimited string, split the
+-- same way the radio tag_overlap arm does (regexp_split_to_table on
+-- [;,]). Same exclusion filters as the other buckets. Returns nothing
+-- when the user has no taste tags yet (cold start), so the caller
+-- redistributes its slots to the other buckets. Stamped 'taste_unheard'.
+-- $1 = user_id, $2 = date string for md5 tiebreak ordering.
+SELECT t.id, t.album_id, t.artist_id
+  FROM tracks t
+  JOIN LATERAL regexp_split_to_table(coalesce(t.genre, ''), '[;,]') AS g_split(g) ON true
+  JOIN taste_profile_tags nt ON nt.user_id = $1 AND trim(g_split.g) = nt.tag
+ WHERE nt.weight > 0
+   AND trim(g_split.g) <> ''
+   AND NOT EXISTS (
+       SELECT 1 FROM play_events pe
+        WHERE pe.user_id = $1
+          AND pe.track_id = t.id
+          AND pe.was_skipped = false
+   )
+   AND NOT EXISTS (
+       SELECT 1 FROM general_likes gl
+        WHERE gl.user_id = $1 AND gl.track_id = t.id
+   )
+   AND NOT EXISTS (
+       SELECT 1 FROM lidarr_quarantine q
+        WHERE q.user_id = $1 AND q.track_id = t.id
+   )
+ GROUP BY t.id, t.album_id, t.artist_id
+ ORDER BY SUM(nt.weight) DESC, md5(t.id::text || $2::text)
+ LIMIT 120;