docs: add JSDoc headers to all externally exported store functions

src/store/lang/getByIso639.js

@@ -1,6 +1,17 @@
 import { GET_BY_ISO639 } from '../../sql/language.js';
 import request from '../request.js';
 
+/**
+ * Returns a language row by its ISO 639 code.
+ *
+ * @param {object} pool - The mssql connection pool.
+ * @returns {Function} An async function that fetches a language by ISO code.
+ *
+ * @example
+ * const fn = getByIso639(pool);
+ * const lang = await fn('de');
+ * // => { id, iso639 }
+ */
 export default function getByIso639(pool) {
   return async (iso639, tx) => {
     const result = await request(pool, tx)