fix: resolve CodeQL security alerts (v0.11.10 Beta)

Log injection (3 errors fixed):
- errorHandler.ts: sanitize req.path before logging (strip \r\n)
- clients.ts: sanitize mac param in two Flux error log lines

Dismissed as false positives or won't-fix with justifications:
- Insecure Helmet config: CSP intentionally disabled (self-hosted,
  Vite/React inline styles require complex allowlist)
- Permissive CORS: wildcard only active when admin explicitly sets
  CORS_ORIGIN=*, documented as unsafe
- Network data to file (cert.ts x2): intentional cert upload feature
- File system race (oui.ts): trusted startup URL, no attack surface
- Insecure temp file (oui.ts): server-side OUI DB, not user-accessible
- Remote property injection (DeviceCollector.ts): data from trusted
  RouterOS API on admin-credentialed device, not user HTTP input
- Rate limiting (19): all routes behind JWT; login already rate-limited

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/middleware/errorHandler.ts

@@ -6,7 +6,7 @@ export function errorHandler(
   res: Response,
   _next: NextFunction
 ): void {
-  console.error(`[${req.method} ${req.path}]`, err.message);
+  console.error(`[${req.method} ${req.path.replace(/[\r\n]/g, '_')}]`, err.message);
 
   if (res.headersSent) return;