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/service/stats.go

@@ -20,6 +20,15 @@ type RealtimeStats struct {
 	LastAccessedAt *time.Time
 }
 
+type MasterRealtimeSnapshot struct {
+	Requests    int64
+	Tokens      int64
+	QPS         int64
+	QPSLimit    int64
+	RateLimited bool
+	UpdatedAt   *time.Time
+}
+
 func NewStatsService(rdb *redis.Client) *StatsService {
 	return &StatsService{rdb: rdb}
 }
@@ -77,3 +86,70 @@ func (s *StatsService) GetMasterRealtimeStats(ctx context.Context, masterID uint
 	}
 	return RealtimeStats{Requests: reqs, Tokens: tokens}, nil
 }
+
+func (s *StatsService) GetMasterRealtimeSnapshot(ctx context.Context, masterID uint) (MasterRealtimeSnapshot, error) {
+	if s == nil || s.rdb == nil {
+		return MasterRealtimeSnapshot{}, fmt.Errorf("redis client is required")
+	}
+	if masterID == 0 {
+		return MasterRealtimeSnapshot{}, fmt.Errorf("master id required")
+	}
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	statsKey := fmt.Sprintf("master:stats:%d", masterID)
+	rateKey := fmt.Sprintf("master:rate:%d", masterID)
+
+	pipe := s.rdb.Pipeline()
+	reqCmd := pipe.Get(ctx, statsKey+":requests")
+	tokCmd := pipe.Get(ctx, statsKey+":tokens")
+	qpsCmd := pipe.HGet(ctx, rateKey, "qps")
+	limitCmd := pipe.HGet(ctx, rateKey, "limit")
+	blockedCmd := pipe.HGet(ctx, rateKey, "blocked")
+	updatedCmd := pipe.HGet(ctx, rateKey, "updated_at")
+	if _, err := pipe.Exec(ctx); err != nil && err != redis.Nil {
+		return MasterRealtimeSnapshot{}, fmt.Errorf("read realtime stats: %w", err)
+	}
+
+	requests := readCmdInt64(reqCmd)
+	tokens := readCmdInt64(tokCmd)
+	qps := readCmdInt64(qpsCmd)
+	limit := readCmdInt64(limitCmd)
+	blocked := readCmdInt64(blockedCmd) == 1
+	updatedAt := readCmdTime(updatedCmd)
+	if limit < 0 {
+		limit = 0
+	}
+
+	return MasterRealtimeSnapshot{
+		Requests:    requests,
+		Tokens:      tokens,
+		QPS:         qps,
+		QPSLimit:    limit,
+		RateLimited: blocked,
+		UpdatedAt:   updatedAt,
+	}, nil
+}
+
+func readCmdInt64(cmd *redis.StringCmd) int64 {
+	if cmd == nil {
+		return 0
+	}
+	v, err := cmd.Int64()
+	if err != nil {
+		return 0
+	}
+	return v
+}
+
+func readCmdTime(cmd *redis.StringCmd) *time.Time {
+	if cmd == nil {
+		return nil
+	}
+	v, err := cmd.Int64()
+	if err != nil || v <= 0 {
+		return nil
+	}
+	tm := time.Unix(v, 0).UTC()
+	return &tm
+}