fix: resolve data race in token.RNG global

Wrap *rand.Rand in a mutex-protected SafeRand type to make the global
RNG safe for concurrent use across goroutines. The previous bare
*rand.Rand caused data races detected by go test -race.

common/token/token.go

@@ -2,10 +2,37 @@ package token
 
 import (
 	"math/rand"
+	"sync"
 	"time"
 )
 
-var RNG = NewRNG()
+// SafeRand is a concurrency-safe wrapper around *rand.Rand.
+type SafeRand struct {
+	mu  sync.Mutex
+	rng *rand.Rand
+}
+
+func NewSafeRand() *SafeRand {
+	return &SafeRand{
+		rng: rand.New(rand.NewSource(time.Now().UnixNano())),
+	}
+}
+
+func (sr *SafeRand) Intn(n int) int {
+	sr.mu.Lock()
+	v := sr.rng.Intn(n)
+	sr.mu.Unlock()
+	return v
+}
+
+func (sr *SafeRand) Uint32() uint32 {
+	sr.mu.Lock()
+	v := sr.rng.Uint32()
+	sr.mu.Unlock()
+	return v
+}
+
+var RNG = NewSafeRand()
 
 // Generate returns an alphanumeric token of specified length
 func Generate(length int) string {
@@ -16,8 +43,3 @@ func Generate(length int) string {
 	}
 	return string(b)
 }
-
-// NewRNG returns a new NewRNG generator
-func NewRNG() *rand.Rand {
-	return rand.New(rand.NewSource(time.Now().UnixNano()))
-}