refactor: 将客服后端重构为宿主可嵌入模块

- 注入数据库、运行时配置、统一响应、文件存储和平台 AI 能力，补充业务读写工具与客户快捷操作契约。

- 移除模块内重复的组织、客户、工单、标签、技能、旧工作流、MCP 和迁移实现，将身份权限与业务主体交由宿主管理。

- 使用 libSQL 重构向量存储，并完善图片消息、访客身份、排队调度、企业微信和支持聊天页面。

- 统一 HTTP、DTO 与 WebSocket 的 snake_case 协议，补齐模块初始化、业务动作和公共载荷等回归测试。

contract/platform_ai.go

@@ -0,0 +1,56 @@
+package contract
+
+import (
+	"context"
+	"net/http"
+)
+
+const (
+	ModelSourcePlatform = "platform"
+	ModelSourceCustom   = "custom"
+)
+
+// PlatformAIConfig is a runtime-only OpenAI-compatible model configuration.
+// Credentials and the signed HTTP client are supplied by the host and are
+// never persisted in Agent Desk tables or returned by dashboard APIs.
+type PlatformAIConfig struct {
+	BaseURL            string
+	APIKey             string
+	ChatEnabled        bool
+	ChatModel          string
+	VisionEnabled      bool
+	VisionModel        string
+	EmbeddingEnabled   bool
+	ModelName          string
+	EmbeddingModel     string
+	EmbeddingDimension int
+	MaxOutputTokens    int
+	TimeoutMS          int
+	MaxRetryCount      int
+	HTTPClient         *http.Client
+}
+
+type PlatformAIStatus struct {
+	Enabled            bool    `json:"enabled"`
+	Balance            float64 `json:"balance"`
+	Currency           string  `json:"currency"`
+	DefaultProvider    string  `json:"default_provider"`
+	DefaultModel       string  `json:"default_model"`
+	ChatEnabled        bool    `json:"chat_enabled"`
+	ChatProvider       string  `json:"chat_provider"`
+	ChatModel          string  `json:"chat_model"`
+	VisionEnabled      bool    `json:"vision_enabled"`
+	VisionModel        string  `json:"vision_model"`
+	EmbeddingEnabled   bool    `json:"embedding_enabled"`
+	EmbeddingModel     string  `json:"embedding_model"`
+	EmbeddingDimension int     `json:"embedding_dimension"`
+	RechargeURL        string  `json:"recharge_url"`
+}
+
+// PlatformAIProvider lets the host resolve the current model source on every
+// request, so changing the system setting takes effect without restarting.
+type PlatformAIProvider interface {
+	ModelSource(context.Context) (string, error)
+	Config(context.Context) (*PlatformAIConfig, error)
+	Status(context.Context) (*PlatformAIStatus, error)
+}