fix: P0-02 prevent login attempt counter race condition

Add atomic Increment method to cache layers:
- L2Cache interface: add Increment method signature
- RedisCache: implement using Redis INCRBY
- L1Cache: implement with mutex-protected counter
- CacheManager: add Increment that updates both L1 and L2

Update incrementFailAttempts to use atomic Increment instead
of Get-Increment-Set pattern, preventing TOCTOU race.

internal/cache/cache_manager.go

@@ -106,3 +106,16 @@ func (cm *CacheManager) GetL1() *L1Cache {
 func (cm *CacheManager) GetL2() L2Cache {
 	return cm.l2
 }
+
+// Increment 原子递增（同时更新L1和L2）
+func (cm *CacheManager) Increment(ctx context.Context, key string, delta int64, ttl time.Duration) (int64, error) {
+	// 先更新L1
+	cm.l1.Increment(key, delta, ttl)
+
+	// 再更新L2
+	if cm.l2 != nil {
+		return cm.l2.Increment(ctx, key, delta, ttl)
+	}
+
+	return cm.l1.Increment(key, 0, 0), nil
+}