feat: periodically checkpoint server/sud logs to disk

Both loggers previously only wrote their JSON log on their end trigger
(shutdown / Stop-or-DONE), losing the whole session/run's data on a
hard crash or power loss. Add a log_interval (config.json, default
300s) that rewrites the same file whole every N seconds in between.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TvgC7oy9MxaA4ZQxXqkNdS

tasks/server_log.py

@@ -16,7 +16,7 @@ class ServerLogTask(ATask):
 	Sud state — useful for verifying controller and heater behaviour
 	outside of a scheduled brew."""
 
-	def __init__(self, tc: APid, heater: AHeater, heater_task, interval, path='./logs', config=None, dt=None):
+	def __init__(self, tc: APid, heater: AHeater, heater_task, interval, path='./logs', config=None, dt=None, log_interval=None):
 		ATask.__init__(self, interval)
 		self.tc = tc
 		self.heater = heater
@@ -24,10 +24,12 @@ class ServerLogTask(ATask):
 		self.path = path
 		self.config = config
 		self.dt = dt
+		self.log_interval = log_interval
 		self._samples = []
 		self._param_events = []
 		self._t0 = time.monotonic()
 		self._run_id = time.strftime("%Y%m%dT%H%M%S", time.localtime())
+		self._last_write = self._t0
 
 	def log_plant_params(self, elapsed, params):
 		self._param_events.append({'t': time.monotonic() - self._t0, 'params': params})
@@ -36,6 +38,13 @@ class ServerLogTask(ATask):
 		while True:
 			if self._should_sample():
 				self._samples.append(self._take_sample())
+			# Periodically checkpoints to disk (same file, overwritten whole
+			# each time - see write()) so a hard crash/power loss only loses
+			# up to log_interval seconds, not the whole session/run.
+			now = time.monotonic()
+			if self.log_interval and now - self._last_write >= self.log_interval:
+				self.write()
+				self._last_write = now
 			await asyncio.sleep(self.interval)
 
 	def _should_sample(self):