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/pkg/model/model.go

@@ -0,0 +1,144 @@
+package model
+
+import "time"
+
+// ChatCompletionRequest 聊天完成请求
+type ChatCompletionRequest struct {
+	Model       string                  `json:"model" binding:"required"`
+	Messages    []ChatMessage           `json:"messages" binding:"required"`
+	Temperature float64                 `json:"temperature,omitempty"`
+	MaxTokens   int                    `json:"max_tokens,omitempty"`
+	TopP        float64                `json:"top_p,omitempty"`
+	Stream      bool                   `json:"stream,omitempty"`
+	Stop        []string               `json:"stop,omitempty"`
+	N           int                     `json:"n,omitempty"`
+	PresencePenalty float64            `json:"presence_penalty,omitempty"`
+	FrequencyPenalty float64           `json:"frequency_penalty,omitempty"`
+	User       string                  `json:"user,omitempty"`
+}
+
+// ChatMessage 聊天消息
+type ChatMessage struct {
+	Role    string `json:"role" binding:"required"`
+	Content string `json:"content" binding:"required"`
+	Name    string `json:"name,omitempty"`
+}
+
+// ChatCompletionResponse 聊天完成响应
+type ChatCompletionResponse struct {
+	ID      string          `json:"id"`
+	Object  string          `json:"object"`
+	Created int64          `json:"created"`
+	Model   string          `json:"model"`
+	Choices []Choice       `json:"choices"`
+	Usage   Usage           `json:"usage"`
+}
+
+type Choice struct {
+	Index        int        `json:"index"`
+	Message      ChatMessage `json:"message"`
+	FinishReason string     `json:"finish_reason"`
+}
+
+type Usage struct {
+	PromptTokens     int `json:"prompt_tokens"`
+	CompletionTokens int `json:"completion_tokens"`
+	TotalTokens      int `json:"total_tokens"`
+}
+
+// CompletionRequest 完成请求
+type CompletionRequest struct {
+	Model       string  `json:"model" binding:"required"`
+	Prompt      string  `json:"prompt" binding:"required"`
+	MaxTokens   int     `json:"max_tokens,omitempty"`
+	Temperature float64 `json:"temperature,omitempty"`
+	TopP        float64 `json:"top_p,omitempty"`
+	Stream      bool    `json:"stream,omitempty"`
+	Stop        []string `json:"stop,omitempty"`
+	N           int     `json:"n,omitempty"`
+}
+
+// CompletionResponse 完成响应
+type CompletionResponse struct {
+	ID      string     `json:"id"`
+	Object  string     `json:"object"`
+	Created int64     `json:"created"`
+	Model   string     `json:"model"`
+	Choices []Choice1 `json:"choices"`
+	Usage   Usage     `json:"usage"`
+}
+
+type Choice1 struct {
+	Text        string `json:"text"`
+	Index       int    `json:"index"`
+	FinishReason string `json:"finish_reason"`
+}
+
+// StreamResponse 流式响应
+type StreamResponse struct {
+	ID      string      `json:"id"`
+	Object  string      `json:"object"`
+	Created int64      `json:"created"`
+	Model   string      `json:"model"`
+	Choices []Delta    `json:"choices"`
+}
+
+type Delta struct {
+	Delta struct {
+		Content string `json:"content,omitempty"`
+		Role    string `json:"role,omitempty"`
+	} `json:"delta"`
+	Index       int    `json:"index"`
+	FinishReason string `json:"finish_reason,omitempty"`
+}
+
+// ErrorResponse 错误响应
+type ErrorResponse struct {
+	Error ErrorDetail `json:"error"`
+}
+
+type ErrorDetail struct {
+	Message string                 `json:"message"`
+	Type    string                 `json:"type"`
+	Code    string                 `json:"code,omitempty"`
+	Param   string                 `json:"param,omitempty"`
+}
+
+// HealthStatus 健康状态
+type HealthStatus struct {
+	Status    string            `json:"status"`
+	Timestamp time.Time         `json:"timestamp"`
+	Services  map[string]bool   `json:"services"`
+}
+
+// Tenant 租户
+type Tenant struct {
+	ID        int64     `json:"id"`
+	Name      string    `json:"name"`
+	Plan      string    `json:"plan"`
+	CreatedAt time.Time `json:"created_at"`
+}
+
+// Budget 预算
+type Budget struct {
+	TenantID     int64   `json:"tenant_id"`
+	MonthlyLimit float64 `json:"monthly_limit"`
+	AlertThreshold float64 `json:"alert_threshold"`
+	CurrentUsage float64 `json:"current_usage"`
+}
+
+// RouteRequest 路由请求
+type RouteRequest struct {
+	Model     string
+	TenantID  int64
+	RouteType string // "primary", "fallback"
+}
+
+// RouteResult 路由结果
+type RouteResult struct {
+	Provider  string
+	Model     string
+	LatencyMs int64
+	Success   bool
+	Error     error
+}