Add Python agent, WebSocket relay, real viewer, enrollment tokens

- WebSocket relay service (FastAPI) bridges agents and viewers
- Python agent with screen capture (mss), input control (pynput),
  script execution, and auto-reconnect
- Windows service wrapper, PyInstaller spec, NSIS installer for
  silent mass deployment (RemoteLink-Setup.exe /S /SERVER= /ENROLL=)
- Enrollment token system: admin generates tokens, agents self-register
- Real WebSocket viewer replaces simulated canvas
- Linux agent binary served from /downloads/remotelink-agent-linux
- DB migration 0002: viewer_token on sessions, enrollment_tokens table
- Sign-up pages cleaned up (invite-only redirect)

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

.env.example

@@ -16,3 +16,7 @@ AUTH_SECRET=change_me_generate_with_openssl_rand_base64_32
 
 # Public URL of the app (used for invite links, etc.)
 NEXT_PUBLIC_APP_URL=http://localhost:3000
+
+# Relay WebSocket server address as seen from the browser (host:port)
+# For production behind a reverse proxy, set to your domain (no port if using 443/80)
+NEXT_PUBLIC_RELAY_URL=localhost:8765