[API] Add Batch Operations Endpoints

[fentz26]

Problem

No bulk operation support:

• Creating 100 tasks = 100 API calls
• Slow and inefficient
• Network overhead

Proposed Solution

Batch Endpoints

POST /v1/batch
Content-Type: application/json

{
    "operations": [
        {"method": "POST", "path": "/v1/tasks", "body": {...}},
        {"method": "POST", "path": "/v1/tasks", "body": {...}},
        {"method": "DELETE", "path": "/v1/tasks/abc123"}
    ]
}

Response:
{
    "results": [
        {"status": 201, "body": {...}},
        {"status": 201, "body": {...}},
        {"status": 204, "body": null}
    ]
}

Specialized Batch Endpoints

POST /v1/tasks/batch
{
    "tasks": [
        {"title": "Task 1", ...},
        {"title": "Task 2", ...}
    ]
}

DELETE /v1/tasks/batch
{
    "ids": ["abc", "def", "ghi"]
}

Acceptance Criteria

• Generic batch endpoint
• Atomic batch option (all or nothing)
• Per-operation results
• Max batch size: 100 operations
• Timeout handling for large batches

References

• Parent Epic: 🔌 Epic: API Improvements #72