Trigger batch processing of claims uploaded via SFTP.

**Use Case:** High-volume processing (100+ claims) or legacy systems without HTTP multipart capability.

**Authentication:** API Key (X-API-Key header)
**Required Scope:** `claims:write`

**Integration Pattern:**
1. Upload claim files to configured SFTP folder
2. Call this endpoint to trigger batch processing
3. Receive session_id for tracking
4. Poll /status/{session_id} for progress
5. Retrieve results from /result/{session_id} when complete

**Request Format:**
No request body required - automatically scans SFTP inbox folder based on tenant configuration.

**Response Format:**
- session_id: Unique batch session identifier for tracking
- batch_total: Total number of batches to process
- current_batch: Current batch being processed
- sep_in_batch: Number of claims in current batch
- status: PENDING | PROCESSING | COMPLETED | FAILED
- temporal_workflow_id: Internal workflow identifier

**Webhook Events:**
- batch.started: When batch processing begins
- batch.completed: When all claims in batch finish processing
- batch.failed: If batch processing encounters errors

**Cancellation:**
- Use Temporal **Cancel** (SDK `WorkflowHandle.cancel()` or the UI "Cancel" button) to stop a batch.
  Finished claims stay in `success_claim_details`; unfinished claims are marked failed at their
  latest step (`failed_at`, `retryable=true`, `reason="cancelled_at_<step>"`, e.g.
  `cancelled_at_medical_grouper`) and appear in `failed_claim_details`.
  The DB is updated, `batch.failed` fires, and the next batch still runs.
- **Terminate** is not supported for clean compensation — it skips all cleanup and leaves claims stuck.

**Error Handling:**
- 400: Invalid SFTP configuration or no files found
- 401: Invalid API key
- 403: API key lacks claims:write scope
- 500: Server error - check SFTP connection

**Example Response:**
```json
{
    "session_id": "sftp-session-20260122-001",
    "batch_total": 3,
    "current_batch": 1,
    "sep_in_batch": 10,
    "status": "PENDING",
    "finished": False,
    "claim_list": [
        {"claim_id": "CLM-001", "files": ["claim.pdf"]},
        {"claim_id": "CLM-002", "files": ["claim.pdf", "lab.pdf"]}
    ],
    "temporal_workflow_id": "wf-temporal-batch-123"
}
```

**Integration Tips:**
- Configure SFTP settings via Admin API before first use
- Batch size configurable (default 10 claims per batch)
- Use webhooks instead of polling for better performance
- Upload files during off-peak hours for optimal processing