feat: on-demand RDP - connect xfreerdp only when session opens

Replaces always-on xfreerdp with on-demand model (load 12 to under 1 at idle).
- rdp-proxy/manager.py: HTTP server port 7001 managing xfreerdp lifecycle
- rdp-proxy/entrypoint.sh: starts Xvfb+x11vnc+websockify+manager, no auto-connect
- rdp-proxy/Dockerfile: adds python3, copies manager.py, exposes 7001
- runtime.py: connect_rdp_slot and disconnect_rdp_slot via manager HTTP API
- terminate_session_record: disconnect instead of container restart
- main.py: calls connect_rdp_slot in background thread on session create
- maintenance.py: cleanup_loop disconnects on expire, run_maintenance_service
  includes RDP slot init, maintenance_runner fixed to import maintenance

app/main.py

@@ -1,6 +1,7 @@
 import datetime as dt
 import logging
 import re
+import threading
 import uuid
 import time
 import contextvars
@@ -46,7 +47,7 @@ from runtime import (
     get_universal_pool_status, get_web_pool_status, LockTimeoutError, open_warm_web_url,
     _rdp_slot_container_name, route_ready, sanitize_client_resolution,
     service_uses_universal_pool, session_redirect_url,
-    start_rdp_slot_container, stop_rdp_slot_container,
+    connect_rdp_slot, start_rdp_slot_container, stop_rdp_slot_container,
     stop_runtime_container, terminate_active_slot_sessions,
     terminate_session_record, wait_for_session_route,
 )
@@ -579,6 +580,7 @@ def go_service(
                     log_event("session_created", user_id=user.id, service_slug=service.slug, session_id=session_id, mode="rdp_slot", slot_id=free_slot.id)
                     audit(db, "SESSION_CREATE_RDP_SLOT", f"service={service.slug} session={session_id} slot={free_slot.id}", user_id=user.id)
                     _emit("session_created_rdp_slot", session_id=session_id, slot_id=free_slot.id)
+                    threading.Thread(target=connect_rdp_slot, args=(free_slot.id,), daemon=True).start()
                     return RedirectResponse(url=f"/s/{session_id}/", status_code=303)
                 else:
                     # Legacy: no slots configured — exclusive single-session behaviour