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/api/__init__.py

@@ -43,6 +43,7 @@ from app.api.shared_links import public_router as shared_links_public_router
 from app.api.shared_links import router as shared_links_router
 from app.api.similarity import router as similarity_router
 from app.api.subscriptions import router as subscriptions_router
+from app.api.translation import router as translation_router
 from app.api.url_upload import router as url_upload_router
 
 # Import all the individual routers
@@ -96,3 +97,4 @@ router.include_router(audit_logs_router)
 router.include_router(i18n_router)
 router.include_router(mobile_router)
 router.include_router(compliance_router)
+router.include_router(translation_router)