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/tools/ipcalc.py

@@ -26,10 +26,13 @@ def ip_calculate():
 
         # Avoid materialising millions of host objects for large networks.
         if prefix == 32:
+            # Single-host route: the address is both network and host.
             total_hosts = 1
             first_host = str(network.network_address)
             last_host = str(network.network_address)
         elif prefix == 31:
+            # RFC 3021 point-to-point: both addresses are usable hosts,
+            # there is no dedicated network or broadcast address.
             total_hosts = 2
             first_host = str(network.network_address)
             last_host = str(network.broadcast_address)
@@ -39,6 +42,8 @@ def ip_calculate():
             last_host = str(ipaddress.IPv4Address(bcast_int - 1))
 
         netmask_int = int(network.netmask)
+        # Python's ~ on an int yields a negative arbitrary-precision value;
+        # mask to 32 bits to get the correct unsigned wildcard address.
         wildcard = str(ipaddress.IPv4Address((~netmask_int) & 0xFFFFFFFF))
         ip_class = "Privat" if network.is_private else "Öffentlich"