Add GetVisibleLibraryMediaCounts, which returns the media item count for
each library visible to a given user in a single GROUP BY query over
media_items. It mirrors the visibility logic in GetUserVisibleLibraries
(libraries default to visible unless an explicit false row exists) so
counts can be resolved in one round-trip instead of N per-library
lookups.
Regenerated sqlc bindings (querier.go, queries.sql.go).