Shut heater off on brew end (DONE) and stop (IDLE)

Adds HeaterTask.shutdown() which switches to open-loop at power 0,
disables TC, and broadcasts ClosedLoop=false to clients so the browser
checkbox unchecks immediately. SudTask fires it via a new set_on_end()
callback whenever state reaches DONE or IDLE, giving both a completed
schedule and a manual Stop the same safe behaviour: stirrer and heater
both off, no heater left running unattended.

Re-enabling after shutdown: check the closed-loop checkbox (sends
ClosedLoop=true → TC re-enabled) or adjust the power slider directly;
the next brew's Play press already sends ClosedLoop=true automatically.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SH2D4LRuCnhLSzoYerAfJX

tasks/sud.py

@@ -110,6 +110,7 @@ class SudTask(ATask):
 		# Saved on pause so resume can restore the effective setpoint even
 		# when the current step has temperature=None (inherits from a prior step).
 		self._paused_temp_soll = None
+		self._on_end = None
 		msg_handler.set_recv_handler(self.recv)
 
 	def apply_plant_params(self, grain_mass, water_mass):
@@ -217,12 +218,17 @@ class SudTask(ATask):
 			'WaitForUser': step.get('user_wait_for_continue', False) if step else None,
 		}}))
 
+	def set_on_end(self, callback):
+		self._on_end = callback
+
 	def on_state_changed(self, value):
 		asyncio.create_task(self.send({'State': str(value)}))
 
 		if value in (SudState.DONE, SudState.IDLE):
 			self.stirrer.set_duty_cycle(1.0)
 			self.stirrer.set_speed(0)
+			if self._on_end:
+				self._on_end()
 
 	def on_user_message_changed(self, value):
 		asyncio.create_task(self.send({'UserMessage': value}))