Paperboy overwrites files with duplicate names

If you accept the name suggestion from paperboy, it will happily overwrite any files that happen to have that name in the library folder. Luckily in this case it was overwriting another copy of the same paper but it would be nice to have overwrite safeguards.

Reproduction:

1. Create multiple copies of a pdf in Inbox folder
2. Move them to library with the same name
3. Watch duplicates vanish 😱.

Look for the file move/rename logic in the codebase, likely in a module handling file operations. The issue is about duplicate name handling during import from the Inbox to the library. Check how filenames are generated and if there's existing collision detection. A test case should be added to verify the fix prevents overwriting.

索引モデルが issue の本文から書いたものです。