Add targeted comments for non-obvious constraints and invariants

- logger.py: note why log path uses abspath(__file__) instead of a relative path
- token.py: note why [7:] slice is safe (startswith already verified)
- ipcalc.py: explain /32 single-host and /31 RFC-3021 point-to-point special
  cases; explain why (~netmask) must be masked with 0xFFFFFFFF (Python ~int
  returns a negative arbitrary-precision value, not a 32-bit unsigned integer)
- notes.py: document the module-level _table_ready flag lifetime; explain why
  tzinfo is stripped before passing datetime to mysql-connector
- admin.py: document the module-level _tables_initialized flag lifetime

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

backend/util/logger.py

@@ -2,6 +2,7 @@ import logging
 import os
 from logging.handlers import RotatingFileHandler
 
+# Absolute path so the log dir is always next to this file, regardless of CWD.
 _LOG_DIR = os.path.join(os.path.dirname(os.path.abspath(__file__)), "..", "logs")
 os.makedirs(_LOG_DIR, exist_ok=True)