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/l1.go

@@ -169,3 +169,44 @@ func (c *L1Cache) Cleanup() {
 		c.removeFromAccessOrder(key)
 	}
 }
+
+// Increment 原子递增（用于登录失败计数器等原子操作场景）
+func (c *L1Cache) Increment(key string, delta int64, ttl time.Duration) int64 {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	var expiration int64
+	if ttl > 0 {
+		expiration = time.Now().Add(ttl).UnixNano()
+	}
+
+	current := int64(0)
+	if item, ok := c.items[key]; ok {
+		if item.Expired() {
+			delete(c.items, key)
+			c.removeFromAccessOrder(key)
+		} else {
+			if v, ok := item.Value.(int64); ok {
+				current = v
+			} else if v, ok := item.Value.(int); ok {
+				current = int64(v)
+			} else if v, ok := item.Value.(float64); ok {
+				current = int64(v)
+			}
+		}
+	}
+
+	newVal := current + delta
+	c.items[key] = &CacheItem{
+		Value:      newVal,
+		Expiration: expiration,
+	}
+
+	if _, exists := c.items[key]; !exists {
+		c.accessOrder = append(c.accessOrder, key)
+	} else {
+		c.updateAccessOrder(key)
+	}
+
+	return newVal
+}