feat(api): default download endpoint to processed file version

- Change default `version` param from 'original' to 'processed' so
  GET /api/files/{id}/download (no param) returns the processed file
- Update docstring to reflect new default
- Add tests: ?version=processed, default→processed, invalid→400
- Fix test_file_download_missing_mime_type to use explicit ?version=original
- Add File Download section to docs/API.md

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/api/files.py

@@ -916,14 +916,14 @@ def download_file(
     request: Request,
     file_id: int,
     db: DbSession,
-    version: str = Query("original", description="original or processed"),
+    version: str = Query("processed", description="'processed' (default) or 'original'"),
 ):
     """
-    Download file (original or processed version) as attachment.
+    Download file (processed or original version) as attachment.
 
     Args:
         file_id: ID of the file
-        version: "original" for tmp file, "processed" for processed file
+        version: "processed" (default) for the post-processing file, "original" for the raw upload
 
     Returns:
         File content as attachment download