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/auth/token.py

@@ -18,7 +18,7 @@ def verify_token():
         logger.warning("🔐 Invalid Bearer header")
         return None
 
-    token = auth_header[7:]
+    token = auth_header[7:]  # len("Bearer ") == 7; safe because startswith is verified above
     try:
         decoded = decode(token, SECRET_KEY, algorithms=["HS256"])
         return decoded