fix(upload): structured error codes for quick-upload, fix duplicate filename crash

- Switch errors from plain strings to { filename, code } objects so the
  frontend can show translated messages instead of raw exception text
- Add UNSUPPORTED_FILE_TYPE error code end-to-end (Java enum → errors.ts
  → de/en/es messages)
- Fix IncorrectResultSizeDataAccessException when a filename exists more
  than once in the DB: use findFirstByOriginalFilename instead of
  findByOriginalFilename in storeDocument()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/en.json

@@ -7,6 +7,7 @@
 	"error_document_no_file": "No file is associated with this document.",
 	"error_file_not_found": "The file could not be found in storage.",
 	"error_file_upload_failed": "The file could not be uploaded.",
+	"error_unsupported_file_type": "This file format is not supported.",
 	"error_user_not_found": "User not found.",
 	"error_import_already_running": "An import is already running. Please wait for it to finish.",
 	"error_unauthorized": "You are not logged in.",