Fix upload path-traversal RCE vector, patch all known-vulnerable deps

Path traversal (CWE-22/CWE-73): event-image, branding (logo/favicon),
and event-attachment uploads built the saved filename from the
client-supplied original filename with no sanitization, and multer's
diskStorage joins that straight into the destination path. A crafted
filename containing `../` sequences could write the uploaded file
anywhere the server process has write access — reachable by any
supervisor-level account, and briefly pre-auth via the branding
uploads during initial /setup. Filenames are now always server-
generated (random bytes + validated extension); the original name is
kept only as display metadata.

Dependencies: express-rate-limit was declared only at the repo root
despite being required directly by backend/src/index.js, so a plain
`cd backend && npm install` (per the deployment doc) would never
install it — moved it into backend/package.json. Bumped next off a
version affected by a critical unauthenticated RCE (React Flight
protocol) and switched it from an exact pin to a caret range so future
patches install automatically. Bumped multer/nodemailer/jsonwebtoken/
uuid to patched versions, with an override forcing the vulnerable
nested uuid inside exceljs and the vulnerable postcss bundled inside
next to the patched versions too. `npm audit` is now clean (0
vulnerabilities) across root, backend, and frontend.

Hardening: jwt.verify() now pins algorithms: ['HS256'] instead of
trusting the token header; /uploads now serves with a restrictive CSP
and X-Content-Type-Options: nosniff so an uploaded SVG containing
<script> can't execute if opened directly.

Verified: backend's Jest suite passes, the backend boots and serves
real requests on the bumped deps, and `next build` compiles/type-
checks cleanly on the bumped frontend deps.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CSWFWQsjTc9GyffPiXEDQT

backend/src/controllers/eventController.js

@@ -3,6 +3,7 @@ const { v4: uuidv4 } = require('uuid');
 const multer = require('multer');
 const path = require('path');
 const fs = require('fs');
+const crypto = require('crypto');
 const { assertEventOpen } = require('../utils/cashupUtils');
 const { logAdminAction } = require('../utils/adminAudit');
 const { getClientIp } = require('../utils/requestUtils');
@@ -987,7 +988,10 @@ const attachmentsStorage = multer.diskStorage({
     }
   },
   filename: function (req, file, cb) {
-    const unique = `${Date.now()}-${file.originalname}`;
+    // Extension only — file.originalname is untrusted and joining it into a
+    // path allows `../` traversal to write outside the upload directory.
+    const ext = path.extname(file.originalname).toLowerCase();
+    const unique = `event-file-${Date.now()}-${crypto.randomBytes(8).toString('hex')}${ext}`;
     cb(null, unique);
   }
 });

backend/src/controllers/uploadController.js

@@ -1,7 +1,15 @@
 const path = require('path');
 const fs = require('fs');
+const crypto = require('crypto');
 const multer = require('multer');
 
+// Builds a filename multer can never be tricked into escaping the upload
+// directory with — extension only, no attacker-controlled path segments.
+// (file.originalname is untrusted; joining it into a path allows `../` traversal.)
+function safeFilename(prefix, ext) {
+    return `${prefix}-${Date.now()}-${crypto.randomBytes(8).toString('hex')}${ext}`;
+}
+
 // Setup multer storage
 const storage = multer.diskStorage({
     destination: function (req, file, cb) {
@@ -24,8 +32,7 @@ const storage = multer.diskStorage({
         }
     },
     filename: function (req, file, cb) {
-        const uniqueName = `${Date.now()}-${file.originalname}`;
-        cb(null, uniqueName);
+        cb(null, safeFilename('event', path.extname(file.originalname).toLowerCase()));
     }
 });
 
@@ -54,7 +61,7 @@ const logoStorage = multer.diskStorage({
         }
     },
     filename: function (req, file, cb) {
-        cb(null, `logo-${Date.now()}${path.extname(file.originalname).toLowerCase()}`);
+        cb(null, safeFilename('logo', path.extname(file.originalname).toLowerCase()));
     }
 });
 
@@ -82,7 +89,7 @@ const faviconStorage = multer.diskStorage({
         }
     },
     filename: function (req, file, cb) {
-        cb(null, `favicon-${Date.now()}${path.extname(file.originalname).toLowerCase()}`);
+        cb(null, safeFilename('favicon', path.extname(file.originalname).toLowerCase()));
     }
 });