feat(api): add realtime stats endpoints for masters

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.

internal/model/models.go

@@ -20,7 +20,7 @@ type Master struct {
 	Epoch            int64  `gorm:"default:1" json:"epoch"`                        // used for revocation/rotation
 	Status           string `gorm:"size:50;default:'active'" json:"status"`        // active, suspended
 	MaxChildKeys     int    `gorm:"default:5" json:"max_child_keys"`
-	GlobalQPS        int    `gorm:"default:3" json:"global_qps"`
+	GlobalQPS        int    `gorm:"default:0" json:"global_qps"`
 }
 
 // Key represents a child access token issued by a Master.