Add backend unit/integration tests, Playwright e2e tests, README, cookie Secure fix

backend/src/routes/admin.ts

@@ -5,8 +5,9 @@ import { pool } from '../db';
 
 const router = Router();
 
+const UPLOADS_DIR = process.env.UPLOADS_DIR || '/app/uploads';
 const storage = multer.diskStorage({
-  destination: '/app/uploads',
+  destination: UPLOADS_DIR,
   filename: (_req, file, cb) => {
     const ext = path.extname(file.originalname);
     cb(null, `${Date.now()}-${Math.round(Math.random() * 1e6)}${ext}`);