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/middleware/authMiddleware.js

@@ -13,8 +13,9 @@ const protect = async (req, res, next) => {
       // Get token from header
       token = req.headers.authorization.split(' ')[1];
 
-      // Verify token
-      const decoded = jwt.verify(token, process.env.JWT_SECRET);
+      // Verify token — pin the algorithm so a token signed with an
+      // unexpected/attacker-chosen algorithm is never accepted.
+      const decoded = jwt.verify(token, process.env.JWT_SECRET, { algorithms: ['HS256'] });
 
       // Get user from the token (exclude password)
       req.user = await prisma.user.findUnique({
@@ -105,7 +106,7 @@ const optionalAuth = async (req, res, next) => {
   }
   try {
     const token = req.headers.authorization.split(' ')[1];
-    const decoded = jwt.verify(token, process.env.JWT_SECRET);
+    const decoded = jwt.verify(token, process.env.JWT_SECRET, { algorithms: ['HS256'] });
     const user = await prisma.user.findUnique({
       where: { id: decoded.id },
       select: { id: true, name: true, email: true, role: true, isActive: true, createdAt: true, updatedAt: true, phoneNumber: true, tokenVersion: true }