Add CURRENCY_API_KEY option to config files

backend/src/__tests__/config.test.ts

@@ -51,6 +51,7 @@ describe('Mempool Backend Config', () => {
         ALLOW_UNREACHABLE: true,
         PRICE_UPDATES_PER_HOUR: 1,
         MAX_TRACKED_ADDRESSES: 1,
+        CURRENCY_API_KEY: ''
       });
 
       expect(config.ELECTRUM).toStrictEqual({ HOST: '127.0.0.1', PORT: 3306, TLS_ENABLED: true });