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.
Add `POST /admin/masters/{id}/keys` allowing admins to issue child keys
on behalf of a master. Introduce an `issued_by` field in the Key model
to audit whether a key was issued by the master or an admin.
Refactor master service to use typed errors for consistent HTTP status
mapping and ensure validation logic (active status, group check) is
shared.
