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/tests/vocab.rs

@@ -33,7 +33,7 @@ async fn term_with_multilingual_labels_round_trips(pool: PgPool) {
 
     let mut tx = db.pool().begin().await.unwrap();
     let term_id = vocab::add_term(
-        &mut *tx,
+        &mut tx,
         &NewTerm {
             vocabulary_id: v.id,
             external_uri: Some("http://vocab.getty.edu/aat/300011914".into()),
@@ -82,7 +82,7 @@ async fn term_with_no_labels_round_trips_empty(pool: PgPool) {
 
     let mut tx = db.pool().begin().await.unwrap();
     let term_id = vocab::add_term(
-        &mut *tx,
+        &mut tx,
         &NewTerm {
             vocabulary_id: v.id,
             external_uri: None,
@@ -127,7 +127,7 @@ async fn resolve_term_checks_vocabulary_membership(pool: PgPool) {
 
     let mut tx = db.pool().begin().await.unwrap();
     let term_id = vocab::add_term(
-        &mut *tx,
+        &mut tx,
         &NewTerm {
             vocabulary_id: material.id,
             external_uri: None,