feat(api): add batch document upload endpoint

[nancysangani]

Closes #435

📝 What does this PR do?

Adds POST /api/v1/documents/upload/batch so users can upload multiple files in a single request. Each file is validated, saved, and dispatched to Celery independently — failures for individual files are recorded in a failed[] list without blocking the rest of the batch.

Changes:

• backend/app/schemas.py — new BatchUploadResponse schema
• backend/app/routes/documents.py — new /upload/batch endpoint with per-file validation, DB persistence, Celery dispatch, and in-process fallback
• backend/tests/test_batch_upload.py — 9 unit tests covering happy paths, partial failures, full failure, auth, chunk validation, Celery fallback, and DB persistence

🗂️ Type of Change

• ✨ New feature
• 🧪 Tests

🧪 How was this tested?

9 tests in backend/tests/test_batch_upload.py:

Test	Asserts
test_batch_upload_single_file	202, 1 document created
test_batch_upload_multiple_files	202, 3 documents created
test_batch_upload_rejects_bad_extension	bad file in failed[], good file accepted
test_batch_upload_all_files_fail_returns_400	400 when nothing succeeds
test_batch_upload_requires_auth	401/403 without token
test_batch_upload_invalid_chunk_size	400 for chunk_size=50
test_batch_upload_invalid_chunk_overlap	400 for overlap >= chunk_size
test_batch_upload_celery_fallback_uses_background_task	task_id starts with local_
test_batch_upload_document_persisted_in_db	DB row has correct fields
test_batch_upload_chunk_settings_stored	custom chunk settings saved

✅ Self-Review Checklist

• Branch based on dev, not main
• No secrets / API keys added
• No changes to main or HuggingFace deployment config
• Follows existing code style (same pattern as /upload)
• New endpoint placed before /urlupload to keep route specificity correct

[nancysangani]

Hi @param20h please review the PR when you get a chance. Thanks!

[github-actions]

🎉 Congratulations on getting your Pull Request merged! 🎉

Thank you for contributing to PDF-Assistant-RAG as part of GSSoC '26! 🚀

Keep up the great work! ✨