Stopping the engine on Windows stops the whole engine

The installer runs the engine as <venv>\Scripts\python.exe, and since
Python 3.7.2 that file is a redirector that spawns the real interpreter
as a child. Stop() terminated the redirector and left the interpreter -
the process holding the cameras and the SQLite WAL - running with no
parent and nothing able to stop it. Seen on a Windows install: Quit from
the tray, and recognition still running.

The child is now started suspended, placed in a job object with
KILL_ON_JOB_CLOSE, and resumed. Cancel terminates the job, so the whole
tree goes; and the job dies with this process, so it goes even if the app
crashes. CREATE_NO_WINDOW while here: python.exe is a console program
and a GUI parent otherwise opens a black console on the shop counter.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KGcjxF1cNLcuwc3DAPcnfj

agent/go.mod

@@ -7,4 +7,5 @@ require (
 	github.com/gorilla/websocket v1.5.0 // indirect
 	golang.org/x/net v0.8.0 // indirect
 	golang.org/x/sync v0.1.0 // indirect
+	golang.org/x/sys v0.20.0 // indirect
 )