HeaterTask.shutdown() switched the heater to open-loop when a brew
ended, but nothing switched it back when a new Sud was started. The
browser happened to send {ClosedLoop: true} as part of its UI setup,
so its connect-and-start path worked; PyQt (and any other client that
doesn't send that message) left the heater in open-loop and the TC
never drove it.
Add SudTask._on_start / set_on_start() hook, HeaterTask.start_closed_loop()
(mirror of shutdown()), and wire them in brewpi.py so the server
switches to closed-loop and broadcasts the new state to all clients the
moment any client starts a brew.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>