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/admin.py

@@ -7,6 +7,9 @@ from util.logger import logger
 admin_bp = Blueprint("admin", __name__)
 
 _VALID_ROLES = {"user", "admin"}
+
+# Module-level flag: DDL runs at most once per process lifetime.
+# Resets automatically on worker restart, which re-triggers the check.
 _tables_initialized = False