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/sud_log.py

@@ -14,8 +14,8 @@ class SudLogTask(ServerLogTask):
 	logs/log_{date_time}_{sud_name}.log. A Pause/resume doesn't start a new
 	file - only a fresh Play (from IDLE/DONE) does."""
 
-	def __init__(self, tc: APid, heater: AHeater, heater_task, sud, interval, path='./logs', config=None, dt=None):
-		ServerLogTask.__init__(self, tc, heater, heater_task, interval, path, config, dt)
+	def __init__(self, tc: APid, heater: AHeater, heater_task, sud, interval, path='./logs', config=None, dt=None, log_interval=None):
+		ServerLogTask.__init__(self, tc, heater, heater_task, interval, path, config, dt, log_interval)
 		self.sud = sud
 		self._active = False
 
@@ -31,6 +31,7 @@ class SudLogTask(ServerLogTask):
 			return
 		self._samples = []
 		self._t0 = time.monotonic()
+		self._last_write = self._t0
 		self._run_id = time.strftime("%Y%m%dT%H%M%S", time.localtime())
 		self._active = True