[ADD] finalize_db: run `_gc_file_store_unsafe` after attachment cleanup

Add ``lib/python/file_gc.py``, a minimal Odoo-shell script that calls
``ir.attachment._gc_file_store_unsafe()`` to remove filestore files
no longer referenced by any ``ir_attachment`` record.

Invoke it in ``finalize_db.sh`` after the SQL attachment purges so
that the deleted CSS/JS asset bundles, resized partner thumbnails,
and orphaned ``ir_attachment`` rows also have their backing files
cleaned from disk.

lib/python/file_gc.py

@@ -0,0 +1 @@
+env['ir.attachment']._gc_file_store_unsafe()