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