feat(files): add file deletion endpoint

[martsokha]

Context

Files have create/list/read but no delete endpoint. Document deletion is needed for data retention enforcement and user-initiated cleanup.

Requirements

• DELETE /files/{fileId}/ endpoint
• Soft delete by default (sets deleted_at timestamp)
• Permanent removal handled by retention enforcement (see data retention issue)
• Authorization: file owner or workspace admin
• Deletion cascades to annotations attached to the file
• Deletion recorded in the activity audit trail

References

• Server docs: ARCHITECTURE.md - Data Model