feat(translation): add model, config, task, API, and UI for default document language translation

- Add detected_language, default_language_text, default_language_code columns to FileRecord
- Add default_document_language column to UserProfile
- Add DEFAULT_DOCUMENT_LANGUAGE config setting (defaults to "en")
- Create translate_to_default_language Celery task
- Integrate translation trigger into embed_metadata_into_pdf pipeline
- Add /api/files/{id}/translate and /api/files/{id}/translation/default API endpoints
- Add /files/{id}/text/default-language view endpoint
- Update file_view.html with translation sections (default language, on-the-fly)
- Add translation keys to en.json
- Create Alembic migration 036
- Update .env.demo

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

app/views/files.py

@@ -832,6 +832,34 @@ def get_processed_text(request: Request, file_id: int, db: Session = Depends(get
         )
 
 
+@router.get("/files/{file_id}/text/default-language")
+@require_login
+def get_default_language_text(request: Request, file_id: int, db: Session = Depends(get_db)):
+    """Return the persisted default-language translation for the file view."""
+    from fastapi import status
+    from fastapi.responses import JSONResponse
+
+    from app.models import FileRecord
+
+    file_record = db.query(FileRecord).filter(FileRecord.id == file_id).first()
+    if not file_record:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=_FILE_NOT_FOUND)
+
+    if not file_record.default_language_text:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="No default-language translation available",
+        )
+
+    return JSONResponse(
+        content={
+            "text": file_record.default_language_text,
+            "language_code": file_record.default_language_code,
+            "detected_language": file_record.detected_language,
+        }
+    )
+
+
 @router.get("/duplicates")
 @require_login
 def duplicates_page(