fix(backend): resolve 3 test suite failures from merge conflicts

- server.js: remove duplicate startIdempotencyCleanup import that caused
  SyntaxError breaking csrf.test.js and server.errorHandler.test.js
- donations.test.js: update error assertion from 'Invalid Stellar public
  key' to 'Invalid Stellar address' to match AppError(INVALID_ADDRESS)

All 67 backend test suites pass (729 tests), lint 0 errors.

Author: scarface-dev1

backend/src/routes/donations.test.js

@@ -226,7 +226,7 @@ describe("POST /api/donations", () => {
 
     expect(next).toHaveBeenCalledTimes(1);
     expect(res.statusCode).toBe(400);
-    expect(res.body.error).toBe("Invalid Stellar public key");
+    expect(res.body.error).toBe("Invalid Stellar address");
     expect(pool.connect).not.toHaveBeenCalled();
   });
 

backend/src/server.js

@@ -69,10 +69,6 @@ const { startIndexer } = require("./services/indexerService");
 const { startReconciler, stopReconciler } = require("./services/indexerReconciler");
 const { startDLQWorker, stopDLQWorker } = require("./services/indexerDLQWorker");
 const lifecycle = require("./services/lifecycle");
-const {
-  start: startIdempotencyCleanup,
-  stop: stopIdempotencyCleanup,
-} = require("./services/idempotencyCleanup");
 
 Sentry.init({
   dsn: process.env.SENTRY_DSN || "",