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 StatsService integration to admin and master handlers,
exposing realtime metrics (requests, tokens, QPS, rate limit status)
via new endpoints:
- GET /admin/masters/:id/realtime
- GET /v1/realtime
Also embed realtime stats in the existing GET /admin/masters/:id
response and change GlobalQPS default to 0 with validation to
reject negative values.
