test(db): cover delete/empty-changes/empty-history; clarify map_row naming

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/db/tests/audit.rs

@@ -68,3 +68,38 @@ async fn history_is_scoped_to_one_entity(pool: PgPool) {
     assert_eq!(only_a.len(), 1);
     assert_eq!(only_a[0].entity_id, a);
 }
+
+#[sqlx::test]
+async fn deleted_action_with_empty_changes_round_trips(pool: PgPool) {
+    let db = Db::from_pool(pool);
+    let id = Uuid::new_v4();
+
+    audit::record(
+        db.pool(),
+        &NewAuditEvent {
+            actor: AuditActor::System,
+            action: AuditAction::Deleted,
+            entity_type: "object".into(),
+            entity_id: id,
+            changes: vec![],
+        },
+    )
+    .await
+    .unwrap();
+
+    let history = audit::history_for(db.pool(), "object", id).await.unwrap();
+    assert_eq!(history.len(), 1);
+    assert_eq!(history[0].action, AuditAction::Deleted);
+    assert!(history[0].changes.is_empty());
+}
+
+#[sqlx::test]
+async fn history_is_empty_for_unknown_entity(pool: PgPool) {
+    let db = Db::from_pool(pool);
+
+    let history = audit::history_for(db.pool(), "object", Uuid::new_v4())
+        .await
+        .unwrap();
+
+    assert!(history.is_empty());
+}