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/dbq/discover.sql.go

@@ -211,3 +211,69 @@ func (q *Queries) ListRandomUnheardTracksForDiscover(ctx context.Context, arg Li
 	}
 	return items, nil
 }
+
+const listTasteUnheardTracksForDiscover = `-- name: ListTasteUnheardTracksForDiscover :many
+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
+`
+
+type ListTasteUnheardTracksForDiscoverParams struct {
+	UserID  pgtype.UUID
+	Column2 string
+}
+
+type ListTasteUnheardTracksForDiscoverRow struct {
+	ID       pgtype.UUID
+	AlbumID  pgtype.UUID
+	ArtistID pgtype.UUID
+}
+
+// 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.
+func (q *Queries) ListTasteUnheardTracksForDiscover(ctx context.Context, arg ListTasteUnheardTracksForDiscoverParams) ([]ListTasteUnheardTracksForDiscoverRow, error) {
+	rows, err := q.db.Query(ctx, listTasteUnheardTracksForDiscover, arg.UserID, arg.Column2)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ListTasteUnheardTracksForDiscoverRow
+	for rows.Next() {
+		var i ListTasteUnheardTracksForDiscoverRow
+		if err := rows.Scan(&i.ID, &i.AlbumID, &i.ArtistID); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}

internal/db/migrations/0041_discover_taste_unheard.down.sql

@@ -0,0 +1,27 @@
+-- Reverse 0041_discover_taste_unheard.up.sql.
+--
+-- Null out any rows stamped with the new value before shrinking the
+-- CHECK back to the 0039 vocabulary, or the ADD CONSTRAINT would fail
+-- on existing 'taste_unheard' rows (mirrors 0039's down).
+UPDATE play_events SET pick_kind = NULL WHERE pick_kind = 'taste_unheard';
+UPDATE playlist_tracks SET pick_kind = NULL WHERE pick_kind = 'taste_unheard';
+
+ALTER TABLE playlist_tracks
+    DROP CONSTRAINT playlist_tracks_pick_kind_check;
+ALTER TABLE playlist_tracks
+    ADD CONSTRAINT playlist_tracks_pick_kind_check
+        CHECK (pick_kind IN (
+            'taste', 'fresh',
+            'dormant', 'cross_user', 'random',
+            'tier1', 'tier2', 'tier3'
+        ));
+
+ALTER TABLE play_events
+    DROP CONSTRAINT play_events_pick_kind_check;
+ALTER TABLE play_events
+    ADD CONSTRAINT play_events_pick_kind_check
+        CHECK (pick_kind IN (
+            'taste', 'fresh',
+            'dormant', 'cross_user', 'random',
+            'tier1', 'tier2', 'tier3'
+        ));

internal/db/migrations/0041_discover_taste_unheard.up.sql

@@ -0,0 +1,32 @@
+-- Discover taste-targeted novelty bucket (milestone #127, Scribe #1252/#1488).
+--
+-- The 2-week metrics review found Discover beating the manual baseline on
+-- skip rate — which for a discovery surface means it is playing it safe:
+-- on a single-user server it is effectively dormant + crude-random, and
+-- the per-user taste profile (n_tags, #796) went unused. This adds a
+-- fourth Discover candidate bucket that ranks unheard tracks by the
+-- user's taste-profile tag weights ("novelty that fits your vibe"), so
+-- its picks need a new pick_kind provenance value.
+--
+-- Postgres CHECK whitelists can't be altered in place: DROP + re-ADD with
+-- the expanded list, in the same migration (family rule — new CHECK-enum
+-- value needs a same-change migration).
+ALTER TABLE playlist_tracks
+    DROP CONSTRAINT playlist_tracks_pick_kind_check;
+ALTER TABLE playlist_tracks
+    ADD CONSTRAINT playlist_tracks_pick_kind_check
+        CHECK (pick_kind IN (
+            'taste', 'fresh',
+            'dormant', 'cross_user', 'random', 'taste_unheard',
+            'tier1', 'tier2', 'tier3'
+        ));
+
+ALTER TABLE play_events
+    DROP CONSTRAINT play_events_pick_kind_check;
+ALTER TABLE play_events
+    ADD CONSTRAINT play_events_pick_kind_check
+        CHECK (pick_kind IN (
+            'taste', 'fresh',
+            'dormant', 'cross_user', 'random', 'taste_unheard',
+            'tier1', 'tier2', 'tier3'
+        ));

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;