feat: add PIN-based login with server-side cookie sessions

- Server: POST /auth/login (verify PIN, set httpOnly cookie)
- Server: GET /auth/check, POST /auth/logout
- Client: Login page shown when not authenticated
- Session persists 7 days via httpOnly cookie
- PIN stored server-side only (ADMIN_PIN env var)
- Dashboard loads data only after successful auth

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/.env.example

@@ -14,3 +14,7 @@ NOCODB_BASE_ID=your-base-id
 
 # ETL sync secret (for cron auth)
 ETL_SECRET=your-secret-here
+
+# Auth
+ADMIN_PIN=your-pin-code
+SESSION_SECRET=your-random-session-secret