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/README.md

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+# RemoteLink Agent
+
+Cross-platform remote support agent. Streams the screen to the RemoteLink server and handles remote mouse/keyboard input and script execution.
+
+## Quick start
+
+### Run-once (portable, no install)
+
+```bash
+# Linux / macOS
+python agent.py --server https://remotelink.example.com --enroll YOUR_TOKEN --run-once
+
+# Windows — run the pre-built exe
+remotelink-agent.exe --server https://remotelink.example.com --enroll YOUR_TOKEN --run-once
+```
+
+### Permanent install (saves config, reconnects on reboot)
+
+```bash
+python agent.py --server https://remotelink.example.com --enroll YOUR_TOKEN
+# Config saved to /etc/remotelink/agent.json (Linux) or C:\ProgramData\RemoteLink\agent.json (Windows)
+# Start normally on next run:
+python agent.py
+```
+
+### Windows service (runs on login / system start)
+
+```batch
+remotelink-agent-service.exe install
+remotelink-agent-service.exe start
+```
+
+### Mass deployment (NSIS installer, silent)
+
+```batch
+RemoteLink-Setup.exe /S /SERVER=https://remotelink.example.com /ENROLL=YOUR_TOKEN
+```
+
+This silently installs, enrolls the machine, installs the Windows service, and starts it — no UI shown.
+
+## Building
+
+```bash
+# Install deps + build portable binary (all platforms)
+./build.sh
+
+# Output: dist/remotelink-agent  (Linux/macOS)
+#         dist/remotelink-agent.exe + dist/remotelink-agent-service.exe  (Windows)
+#         RemoteLink-Setup.exe  (Windows + NSIS installed)
+```
+
+## Enrollment tokens
+
+Generate enrollment tokens in the RemoteLink web UI under **Admin → Enrollment Tokens**.
+
+Each token can optionally have:
+- An expiry date
+- A max-uses limit
+- A label for tracking
+
+## Protocol
+
+The agent connects to the relay WebSocket server:
+- `ws://<relay-host>:8765/ws/agent?machine_id=<id>&access_key=<key>`
+
+It streams JPEG frames as binary WebSocket messages and receives JSON control events.