feat(db): add object.fields jsonb column, read it into CatalogueObject

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/db/tests/catalog.rs

@@ -116,3 +116,16 @@ async fn object_with_date_and_all_none_optionals_round_trips(pool: PgPool) {
             .any(|c| c.field == "recording_date")
     );
 }
+
+#[sqlx::test]
+async fn new_object_has_empty_fields(pool: PgPool) {
+    let db = Db::from_pool(pool);
+    let mut tx = db.pool().begin().await.unwrap();
+    let id = catalog::create_object(&mut tx, AuditActor::System, &sample_input("LM-9"))
+        .await
+        .unwrap();
+    tx.commit().await.unwrap();
+
+    let obj = catalog::object_by_id(db.pool(), id).await.unwrap().unwrap();
+    assert_eq!(obj.fields, serde_json::json!({}));
+}