Avoid importing ctypes at import time in django.core.files.locks on Windows

[Alexandre Zollinger Chohfi]

On Windows, django/core/files/locks.py imports ctypes and ctypes.wintypes and defines the OVERLAPPED structure and the LockFileEx/UnlockFileEx prototypes at module level. The module is imported by django.core.cache.backends.filebased, which django.core.checks imports, so every import django.db.models on Windows imports ctypes and loads _ctypes.pyd, libffi and the OLE DLLs _ctypes links against, whether or not anything ever locks a file. On other platforms the module only imports fcntl.

Measured on Windows 11 with Python 3.14.4 and Django main (8e3dec6):

• python -X importtime -c "import django.db.models": django.core.files.locks takes 11.2 ms cumulative on Windows (0.1 ms on Linux, same machine under WSL2).
• Building the ctypes objects on the first lock()/unlock() call instead: import django.db.models is 6.0 ms faster (310.7 -> 300.9 ms, 95% CI 1.2-13.6 ms, 60 alternating runs timed inside the process).

The constants and the lock/unlock functions stay at module level, so the public behavior is unchanged. runtests.py files cache file_storage passes on Windows with the change.

I'd be happy to submit the patch.

AI disclosure: the patch was prepared with GitHub Copilot (Claude Opus 5.5 model), which was used to find the import, and write the patch.