Fix all backend lint errors and security vulnerabilities

Backend lint (4 errors fixed):
- auth.ts: suppress @typescript-eslint/no-namespace on Express global
  augmentation (legitimate TS pattern for extending Request type)
- clients.ts: remove unnecessary \. escape in regex character class
- devices.ts: remove unnecessary \- escape by moving hyphen to front [-:]
- PollerService.ts: add comment to empty catch block

Security audit (high/critical CVEs fixed):
- frontend: bump axios (critical SSRF), lodash (high injection),
  follow-redirects (moderate) via npm audit fix
- backend: bump nodemailer 6.x → 8.0.5 (fixes 4 SMTP injection CVEs)

backend/src/middleware/auth.ts

@@ -8,6 +8,7 @@ export interface AuthPayload {
 }
 
 declare global {
+  // eslint-disable-next-line @typescript-eslint/no-namespace
   namespace Express {
     interface Request {
       user?: AuthPayload;