test(db): zero-label authority round-trip; doc the labels-json constant

Also fix pre-existing clippy::explicit_auto_deref in all db test files
(&mut *tx → &mut tx across authority.rs and vocab.rs).

crates/db/src/authority.rs

@@ -3,6 +3,7 @@
 use domain::{Authority, AuthorityId, AuthorityKind, AuthorityRef, LocalizedLabel, NewAuthority};
 use sqlx::Row;
 
+/// Labels aggregated per row as JSON, to read an authority and its labels in one query.
 const LABELS_JSON: &str = "COALESCE(json_agg(json_build_object('lang', al.lang, 'label', al.label) \
      ORDER BY al.lang) FILTER (WHERE al.authority_id IS NOT NULL), '[]'::json)";