feat(gateway): 实现网关核心模块

实现内容：
- internal/adapter: Provider Adapter抽象层和OpenAI实现
- internal/router: 多Provider路由(支持latency/weighted/availability策略)
- internal/handler: OpenAI兼容API端点(/v1/chat/completions, /v1/completions)
- internal/ratelimit: Token Bucket和Sliding Window限流器
- internal/alert: 告警系统(支持邮件/钉钉/飞书)
- internal/config: 配置管理
- pkg/error: 完整错误码体系
- pkg/model: API请求/响应模型

PRD对齐：
- P0-1: 统一API接入 ✅ (OpenAI兼容)
- P0-2: 基础路由与稳定性 ✅ (多Provider路由+Fallback)
- P0-4: 预算与限流 ✅ (Token Bucket限流)

注意：需要供应链模块支持后再完善成本归因和账单导出

gateway/internal/config/config.go

@@ -0,0 +1,162 @@
+package config
+
+import (
+	"os"
+	"time"
+)
+
+// Config 网关配置
+type Config struct {
+	Server   ServerConfig
+	Database DatabaseConfig
+	Redis    RedisConfig
+	Router   RouterConfig
+	RateLimit RateLimitConfig
+	Alert    AlertConfig
+	Providers []ProviderConfig
+}
+
+// ServerConfig 服务配置
+type ServerConfig struct {
+	Host string
+	Port int
+	ReadTimeout time.Duration
+	WriteTimeout time.Duration
+	IdleTimeout time.Duration
+}
+
+// DatabaseConfig 数据库配置
+type DatabaseConfig struct {
+	Host     string
+	Port     int
+	User     string
+	Password string
+	Database string
+	MaxConns int
+}
+
+// RedisConfig Redis配置
+type RedisConfig struct {
+	Host     string
+	Port     int
+	Password string
+	DB       int
+	PoolSize int
+}
+
+// RouterConfig 路由配置
+type RouterConfig struct {
+	Strategy        string // "latency", "cost", "availability", "weighted"
+	Timeout         time.Duration
+	MaxRetries      int
+	RetryDelay      time.Duration
+	HealthCheckInterval time.Duration
+}
+
+// RateLimitConfig 限流配置
+type RateLimitConfig struct {
+	Enabled       bool
+	Algorithm     string // "token_bucket", "sliding_window", "fixed_window"
+	DefaultRPM    int    // 请求数/分钟
+	DefaultTPM    int    // Token数/分钟
+	BurstMultiplier float64
+}
+
+// AlertConfig 告警配置
+type AlertConfig struct {
+	Enabled    bool
+	Email      EmailConfig
+	DingTalk   DingTalkConfig
+	Feishu     FeishuConfig
+}
+
+// EmailConfig 邮件配置
+type EmailConfig struct {
+	Enabled  bool
+	Host     string
+	Port     int
+	Username string
+	Password string
+	From     string
+	To       []string
+}
+
+// DingTalkConfig 钉钉配置
+type DingTalkConfig struct {
+	Enabled bool
+	WebHook string
+	Secret  string
+}
+
+// FeishuConfig 飞书配置
+type FeishuConfig struct {
+	Enabled bool
+	WebHook string
+	Secret  string
+}
+
+// ProviderConfig Provider配置
+type ProviderConfig struct {
+	Name    string
+	Type    string // "openai", "anthropic", "google", "custom"
+	BaseURL string
+	APIKey  string
+	Models  []string
+	Priority int
+	Weight   float64
+}
+
+// LoadConfig 加载配置
+func LoadConfig(path string) (*Config, error) {
+	// 简化实现，实际应使用viper或类似库
+	cfg := &Config{
+		Server: ServerConfig{
+			Host:         getEnv("GATEWAY_HOST", "0.0.0.0"),
+			Port:         8080,
+			ReadTimeout:  30 * time.Second,
+			WriteTimeout: 30 * time.Second,
+			IdleTimeout:  120 * time.Second,
+		},
+		Router: RouterConfig{
+			Strategy:        "latency",
+			Timeout:         30 * time.Second,
+			MaxRetries:      3,
+			RetryDelay:      1 * time.Second,
+			HealthCheckInterval: 10 * time.Second,
+		},
+		RateLimit: RateLimitConfig{
+			Enabled:       true,
+			Algorithm:     "token_bucket",
+			DefaultRPM:    60,
+			DefaultTPM:    60000,
+			BurstMultiplier: 1.5,
+		},
+		Alert: AlertConfig{
+			Enabled: true,
+			Email: EmailConfig{
+				Enabled: false,
+				Host:   getEnv("SMTP_HOST", "smtp.example.com"),
+				Port:   587,
+			},
+			DingTalk: DingTalkConfig{
+				Enabled: getEnv("DINGTALK_ENABLED", "false") == "true",
+				WebHook: getEnv("DINGTALK_WEBHOOK", ""),
+				Secret:  getEnv("DINGTALK_SECRET", ""),
+			},
+			Feishu: FeishuConfig{
+				Enabled: getEnv("FEISHU_ENABLED", "false") == "true",
+				WebHook: getEnv("FEISHU_WEBHOOK", ""),
+				Secret:  getEnv("FEISHU_SECRET", ""),
+			},
+		},
+	}
+
+	return cfg, nil
+}
+
+func getEnv(key, defaultValue string) string {
+	if value := os.Getenv(key); value != "" {
+		return value
+	}
+	return defaultValue
+}