fix: replace Unicode chars in log messages, fix DeviceInfo.serial, UTF-8 file log

- Replace all Unicode arrows/checkmarks (->  [OK]  [FAIL]) in ach_server.py
  and client.py log calls — Windows cp1252 console can't encode them
- Fix DeviceInfo attribute: serial_number -> serial
- Fix _device_info_to_dict key: serial_number -> serial
- Demote count_events 1E/1F per-key log lines from WARNING to DEBUG
  (they were flooding the console on devices with many stored events)
- FileHandler now opens with encoding='utf-8' so session log files
  can hold any characters without codec errors

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

bridges/ach_server.py

@@ -136,7 +136,7 @@ class AchSession:
         raw_path = session_dir / f"raw_rx_{ts}.bin"
 
         # Wire up a file handler so every protocol log line goes to the session log
-        fh = logging.FileHandler(log_path)
+        fh = logging.FileHandler(log_path, encoding="utf-8")
         fh.setFormatter(logging.Formatter("%(asctime)s  %(levelname)-7s  %(name)s  %(message)s"))
         root_logger = logging.getLogger()
         root_logger.addHandler(fh)