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/migrate.rs

@@ -71,3 +71,16 @@ async fn migrate_creates_field_definition_tables(pool: PgPool) {
         );
     }
 }
+
+#[sqlx::test]
+async fn migrate_adds_object_fields_column(pool: PgPool) {
+    let db = Db::from_pool(pool);
+    let exists: Option<bool> = sqlx::query_scalar(
+        "SELECT true FROM information_schema.columns \
+         WHERE table_name = 'object' AND column_name = 'fields'",
+    )
+    .fetch_optional(db.pool())
+    .await
+    .unwrap();
+    assert_eq!(exists, Some(true));
+}