server: always write log_{date_time}.json for the full server session

Adds ServerLogTask (tasks/server_log.py) which records temp/power samples
from startup to shutdown regardless of Sud state, in the same Name/Samples
JSON format as SudLogTask so analyze_log.py and replay_sim.py can consume
it directly.  The file is written on graceful exit (Ctrl-C) from brewpi.py's
finally block, just before the heater is closed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tasks/__init__.py

@@ -6,3 +6,4 @@ from tasks.pot import PotTask
 from tasks.tempctrl import TcTask
 from tasks.sud import SudTask
 from tasks.sud_log import SudLogTask
+from tasks.server_log import ServerLogTask

tasks/server_log.py

@@ -0,0 +1,52 @@
+import asyncio
+import json
+import os
+import time
+from tasks import ATask
+from components import APid, AHeater
+
+
+class ServerLogTask(ATask):
+	"""Continuously records temp/power samples from server start to shutdown.
+
+	Writes a single log_{date_time}.json on close() in the same format as
+	SudLogTask (Name/Samples) so analyze_log.py and replay_sim.py can
+	consume it directly.  Unlike SudLogTask, which only records during an
+	active Sud run, this log covers the full server session regardless of
+	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'):
+		ATask.__init__(self, interval)
+		self.tc = tc
+		self.heater = heater
+		self.heater_task = heater_task
+		self.path = path
+		self._samples = []
+		self._t0 = time.monotonic()
+		self._run_id = time.strftime("%Y%m%dT%H%M%S", time.localtime())
+
+	async def on_process(self):
+		while True:
+			power_set = max(self.heater_task.power_soll, self.heater_task.power_actor)
+			self._samples.append({
+				't':         time.monotonic() - self._t0,
+				'timestamp': time.time(),
+				'temp_ist':  self.tc.theta_ist,
+				'temp_soll': self.tc.theta_soll_set,
+				'rate_ist':  self.tc.heatrate_ist,
+				'rate_soll': self.tc.heatrate_soll,
+				'power_set': power_set,
+				'power_eff': self.heater.power_eff,
+			})
+			await asyncio.sleep(self.interval)
+
+	def write(self):
+		if not self._samples:
+			return
+		os.makedirs(self.path, exist_ok=True)
+		filename = 'log_{}.json'.format(self._run_id)
+		path = os.path.join(self.path, filename)
+		with open(path, 'w') as f:
+			json.dump({'Name': '', 'Samples': self._samples}, f, indent='\t')
+		print('Server log: wrote {} samples to {}'.format(len(self._samples), filename))