feat: add HTTP/SOCKS5 proxy support for API endpoints

- Add proxy field to ProviderEndpoint database model with migration
- Add ProxyConfig Pydantic model for proxy URL validation
- Extend HTTP client pool with create_client_with_proxy method
- Integrate proxy configuration in chat_handler_base.py and cli_handler_base.py
- Update admin API endpoints to support proxy configuration CRUD
- Add proxy configuration UI in frontend EndpointFormDialog

Fixes #28

frontend/src/api/endpoints/endpoints.ts

@@ -1,6 +1,15 @@
 import client from '../client'
 import type { ProviderEndpoint } from './types'
 
+/**
+ * 代理配置类型
+ */
+export interface ProxyConfig {
+  url: string
+  username?: string
+  password?: string
+}
+
 /**
  * 获取指定 Provider 的所有 Endpoints
  */
@@ -38,6 +47,7 @@ export async function createEndpoint(
     rate_limit?: number
     is_active?: boolean
     config?: Record<string, any>
+    proxy?: ProxyConfig
   }
 ): Promise<ProviderEndpoint> {
   const response = await client.post(`/api/admin/endpoints/providers/${providerId}/endpoints`, data)
@@ -63,6 +73,7 @@ export async function updateEndpoint(
     rate_limit: number
     is_active: boolean
     config: Record<string, any>
+    proxy: ProxyConfig
   }>
 ): Promise<ProviderEndpoint> {
   const response = await client.put(`/api/admin/endpoints/${endpointId}`, data)