## Summary
Use `DocumentService.RemoveDocumentKeepFile` when deleting files that
are linked to documents.
## Change
- inject `DocumentService` into `FileService`
- replace direct document deletion in `deleteSingleFile`
- remove the obsolete file-local engine deletion helper
## Result
Deleting a file now cleans up linked documents through the same service
path used elsewhere, keeping KB counters and document engine cleanup
consistent.
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