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>

db/migrations/0002_agent_relay.sql

@@ -0,0 +1,19 @@
+-- Migration 0002: relay support
+-- Adds viewer_token to sessions and enrollment_tokens table
+
+ALTER TABLE sessions
+  ADD COLUMN IF NOT EXISTS viewer_token UUID DEFAULT gen_random_uuid();
+
+CREATE TABLE IF NOT EXISTS enrollment_tokens (
+  id           UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  token        UUID UNIQUE NOT NULL DEFAULT gen_random_uuid(),
+  label        TEXT,
+  created_by   UUID REFERENCES users(id) ON DELETE SET NULL,
+  created_at   TIMESTAMPTZ NOT NULL DEFAULT now(),
+  expires_at   TIMESTAMPTZ,
+  used_count   INTEGER NOT NULL DEFAULT 0,
+  max_uses     INTEGER,
+  revoked_at   TIMESTAMPTZ
+);
+
+CREATE INDEX IF NOT EXISTS idx_enrollment_tokens_token ON enrollment_tokens(token);