Add POST /documents/upload/batch endpoint for issue 435

[parthpatidar03]

Summary

Closes #435

Adds a new POST /documents/upload/batch endpoint that allows users to upload multiple PDF/DOCX/TXT/MD files simultaneously. Each file is validated, saved, and enqueued as an independent Celery task for parallel RAG ingestion — without blocking the API response.

---

Changes

backend/app/routes/documents.py

• Added POST /documents/upload/batch endpoint
  • Accepts files: List[UploadFile] via multipart/form-data
  • Enforces a max batch size of 20 files per request
  • Each file goes through the existing validate_upload() pipeline (extension → size → MIME → deep parse)
  • Per-file error isolation: one invalid file does not abort the rest of the batch
  • Enqueues a process_document.delay() Celery task per file
  • Falls back to BackgroundTasks if Celery is unavailable (consistent with single upload)
  • Returns 202 Accepted immediately with per-file results

backend/app/schemas.py

• Added BatchUploadResult schema — per-file outcome with filename, success, optional document, optional error
• Added BatchUploadResponse schema — wraps results list with total, succeeded, failed counters

backend/tests/test_batch_upload.py

• New test file with 5 test cases:
  • Unauthenticated request → 401
  • Missing files field → 422
  • More than 20 files → 400
  • Single file happy path → 202, succeeded: 1
  • Partial failure (one good + one bad file) → 202, succeeded: 1, failed: 1
  • Celery unavailable → falls back, task_id prefixed local_
  • Unsupported file extension counted as failure

---

API Reference

Request

POST /documents/upload/batch
Content-Type: multipart/form-data
Authorization: Bearer <token>

files: file1.pdf
files: file2.pdf
...

Response 202 Accepted

{
  "total": 2,
  "succeeded": 2,
  "failed": 0,
  "results": [
    {
      "filename": "file1.pdf",
      "success": true,
      "document": {
        "id": "abc123",
        "original_name": "file1.pdf",
        "status": "pending",
        "task_id": "celery-task-uuid",
        ...
      },
      "error": null
    },
    {
      "filename": "file2.pdf",
      "success": true,
      "document": { ... },
      "error": null
    }
  ]
}

Error cases

Condition	Status
Not authenticated	401
files field missing	422
More than 20 files	400
Individual file invalid	Counted in failed, rest continue

---

Testing

cd backend
pytest tests/test_batch_upload.py -v

[parthpatidar03]

@param20h plz check the pr