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>

agent/requirements.txt

@@ -0,0 +1,10 @@
+# Core
+httpx==0.27.2
+websockets==13.1
+mss==9.0.2
+Pillow==11.0.0
+pynput==1.7.7
+
+# Windows service support (Windows only)
+pywin32==308; sys_platform == "win32"
+pywin32-ctypes==0.2.3; sys_platform == "win32"