BSR販売数推定

Integrate the "BSR Sales Estimate" endpoint from Ecommerce Data API into the current repository. Make the changes directly instead of only returning sample code.

API documentation: https://ecommercedataapi.com/en/docs/amazon/sales/prediction/bsr
Endpoint: POST https://ecommercedataapi.com/v1/amazon/sales/prediction/bsr
API key environment variable: ECOMMERCE_DATA_API_KEY

Example request:
{
  "marketplace": "US",
  "bsr": 1,
  "categoryId": "Category ID"
}

Requirements:
1. Read the API documentation first, then inspect the repository's language, framework, HTTP client, directory structure, and environment-variable conventions.
2. Reuse the existing API client and error-handling patterns. If none exist, add the smallest reusable Ecommerce Data API client and an endpoint-specific wrapper.
3. Read the key from the server-side ECOMMERCE_DATA_API_KEY environment variable and send Content-Type: application/json plus the X-API-Key header. Never write a real key into source code, browser code, logs, test snapshots, or Git.
4. If the environment variable is missing, tell me to configure ECOMMERCE_DATA_API_KEY myself. Do not request, print, or invent the real key, and continue implementing code and tests that do not require a live request.
5. Implement request and response types from the documentation. Handle non-2xx responses, request_id, error.code, error.message, and 429 rate limits.
6. Add a minimal usage example or test that follows the repository's conventions. Run one minimal live request only when the key is already configured and external requests are allowed; otherwise use mocks or static validation.
7. Run the relevant formatter, type checker, and tests. Report changed files, usage, verification results, and any environment variable I still need to configure.