fix(file): use path.Join instead of filepath.Join for object storage keys (#629)

[teddiesloco]

Summary

Fixes #629.

UploadLoopFile and UploadFileForServer were using filepath.Join to construct object-storage keys. On Windows platforms, this injected backslashes (\) into S3/object storage keys instead of the standard portable forward slashes (/).

This PR:

• Replaces filepath.Join with URL/slash-oriented path.Join for object-storage key formatting in backend/modules/foundation/domain/file/service/server.go.
• Ensures key path format is consistent across Windows, macOS, and Linux.

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