Get real-time processing status for an SFTP batch session.

**Use Case:** Track progress of bulk claim submissions uploaded via SFTP.

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

**Integration Pattern:**
1. Upload files to SFTP folder
2. Call POST /claims/sftp/submit to trigger processing
3. Poll this endpoint for progress updates
4. Use returned session_id to track batch

**Response Fields:**
- batch_id (optional): Filter results for a specific batch
- session_id: Unique batch identifier
- status: PENDING | PROCESSING | COMPLETED | COMPLETED_WITH_ERRORS
- batch_total: Total number of batches
- current_batch: Currently processing batch number
- sep_in_batch: Number of claims in current batch
- finished: Boolean indicating if processing is complete
- temporal_workflow_id: Internal workflow tracking ID
- claim_list: Array of claims with their processing status

**Polling Strategy:**
- Poll every 10-30 seconds while status is PROCESSING
- Stop polling when status is COMPLETED or FAILED
- Alternative: Configure webhooks to avoid polling

**Webhook Alternative:**
Configure webhook to receive batch.completed event automatically.

**Example Response:**
```json
{
    "session_id": "sftp-session-20260122-001",
    "status": "PROCESSING",
    "batch_total": 3,
    "current_batch": 2,
    "sep_in_batch": 10,
    "finished": False,
    "temporal_workflow_id": "wf-temporal-batch-123",
    "claim_list": [
        {
            "claim_id": "CLM-001",
            "status": "COMPLETED",
            "files": ["claim.pdf"]
        },
        {
            "claim_id": "CLM-002",
            "status": "PROCESSING",
            "files": ["claim.pdf", "lab.pdf"]
        }
    ]
}
```

**Error Responses:**
- 401: Invalid API key
- 403: API key lacks claims:read scope
- 404: Session ID not found
- 500: Server error retrieving status

**Integration Tips:**
- Use exponential backoff for polling (10s, 20s, 30s)
- Cache session_id for correlation with your system
- Configure webhooks to eliminate polling overhead