feat: add store (JSON persistence) and Enable Banking API client

store.js: read/write ./data/store.json; holds config, tokens, agreements,
requisitions, account mappings.

eb-client.js: RS256 JWT auth (cached, refreshed 60s before expiry),
wrappers for /aspsps, /auth, /sessions, /accounts/* endpoints.

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

src/store.js

@@ -0,0 +1,62 @@
+/**
+ * Persistent JSON store — backed by ./data/store.json (Docker volume mount).
+ *
+ * Schema:
+ * {
+ *   config: { eb_app_id, eb_private_key, gc_secret_id, gc_secret_key } | null,
+ *   gc_tokens: { [token]: { type: "access"|"refresh", expires_at } },
+ *   institutions_cache: [...] | null,
+ *   institutions_cache_at: 0,
+ *   agreements: { [gc_id]: { id, created, institution_id, max_historical_days,
+ *                             access_valid_for_days, access_scope } },
+ *   requisitions: { [gc_id]: { id, created, status, institution_id, agreements,
+ *                               accounts, reference, redirect, link,
+ *                               eb_auth_id, eb_session_id } },
+ *   accounts: { [gc_id]: { gc_id, eb_uid, institution_id, iban, currency,
+ *                           owner_name, name, product, created, last_accessed } }
+ * }
+ */
+
+import { readFileSync, writeFileSync, mkdirSync, existsSync } from 'fs';
+import { dirname } from 'path';
+
+const STORE_PATH = process.env.STORE_PATH || './data/store.json';
+
+const EMPTY = () => ({
+  config: null,
+  gc_tokens: {},
+  institutions_cache: null,
+  institutions_cache_at: 0,
+  agreements: {},
+  requisitions: {},
+  accounts: {},
+});
+
+let _store = null;
+
+export function load() {
+  if (!existsSync(STORE_PATH)) return EMPTY();
+  try {
+    return JSON.parse(readFileSync(STORE_PATH, 'utf8'));
+  } catch {
+    return EMPTY();
+  }
+}
+
+export function get() {
+  if (!_store) _store = load();
+  return _store;
+}
+
+export function save() {
+  const dir = dirname(STORE_PATH);
+  mkdirSync(dir, { recursive: true });
+  writeFileSync(STORE_PATH, JSON.stringify(_store ?? EMPTY(), null, 2));
+}
+
+/** Apply a mutation function and persist immediately. */
+export function update(fn) {
+  const s = get();
+  fn(s);
+  save();
+}