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.
Add unit tests for alert-related functionality:
- alert_handler_test.go: tests for threshold CRUD operations,
alert creation with traffic_spike type, filtering, and stats
- alert_detector_test.go: tests for threshold config loading,
traffic spike severity calculation, deduplication logic,
error rate severity, and nil-safety checks
Also fix format string issues:
- Use %d instead of %.2f for integer QPS in alert messages
- Wrap error description with format directive to avoid linter warning
