Add response envelope middleware to standardize JSON responses as
`{code,data,message}` with consistent business codes across endpoints.
Update Swagger annotations and tests to reflect the new response shape.
BREAKING CHANGE: API responses are now wrapped in a response envelope; clients must read payloads from `data` and handle `code`/`message` fields.
Introduce internal endpoint for flushing accumulated APIKey statistics
from data plane to control plane database, updating both individual
API keys and their parent provider groups with request counts and
success/failure rates.
Add admin endpoint to retrieve aggregated API key statistics summary
across all provider groups, including total requests, success/failure
counts, and calculated rates.
