refactor(packaging): Use SQLAlchemy event for Filename registry (Fixes #576)

[Delta17920]

Fixes #576.

This PR refactors the logic for adding new filenames to the file_registry (Filename model).

As suggested by the issue, this logic was previously handled inline in the file_upload view, which is not ideal. This PR moves the logic to a SQLAlchemy event listener.

Changes:

1. Removed the request.db.add(Filename(filename=filename)) call from warehouse/forklift/legacy.py.
2. Added an @event.listens_for(File, "after_insert") hook to warehouse/packaging/models.py.

This new hook automatically inserts the File.filename into the file_registry table when a new File is created.

To resolve the SAWarning about modifying a session during a flush, the hook uses a direct connection.execute() operation.

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Proof / Test Output:

The existing tests in tests/unit/forklift/test_legacy.py still pass, which confirms the filename logging logic was successfully moved.

[di]

Thanks! This seems fine to me, I'll let @dstufft review as well though.