Bulk file revision: reuse existing files without exposing fileMapping to website users

[anna-parker]

Submission flow:

• user prepares metadata tsv, consensus sequences in fasta and folder with files (one subfolder per submissionId)
• under the hood (not seen to users backend creates a fileMapping)

Revision flow:

• user downloads original data (metadata and sequences)
• user uploads original data with revisions to revision page on website (no file changes or just prepares files to edit potentially recreates folder)
• website gets fileMapping and populates with existing fileMapping, users can delete and add on page or upload a full new folder

created with claude

Backend (/{organism}/get-file-mapping):

• New POST endpoint in SubmissionController that accepts {accessions: [...]} and returns Map<Accession,
  FileCategoryFilesMap>
• New getFileMappingForAccessions method in SubmissionDatabaseService that validates the user can edit these
  accessions, then queries the latest version of each accession and extracts the submitted file mapping
• Description constant GET_FILE_MAPPING_DESCRIPTION added to SubmissionControllerDescriptions

Website (bulk revision pre-population):

• backendClient.ts: new getFileMapping(token, organism, accessions) method
• backend.ts: new filesByAccession schema/type for the response
• FormOrUploadWrapper.tsx: new onMetadataFileChange prop that fires when the metadata file is selected
• DataUploadForm.tsx: when revising, parses the uploaded metadata TSV to extract accession/id column pairs, calls the
  file-mapping endpoint, maps the accession-keyed response to submission-ID-keyed, and pre-populates fileMapping state;
  uses a fileMappingKey to force ExtraFilesUpload to re-mount with the fresh mapping
• FolderUploadComponent.tsx (bulk mode):
  • Delete per file: each file in each submission now has a "Discard" button
  • Add files per submission: each submission group has an "Add files" button with a hidden file input
  • Fixed handleDiscardFile to correctly remove only the specific submission when it becomes empty, rather than
    clearing all submissions

PR Checklist

• All necessary documentation has been adapted.
• The implemented feature is covered by appropriate, automated tests.
• Any manual testing that has been done is documented (i.e. what exactly was tested?)

🚀 Preview: https://claude-no-filemapping.loculus.org

[claude]

This PR may be related to: #5820