feat(update): 添加在线更新和定价数据获取的代理支持

针对国内服务器访问 GitHub 困难的问题，为在线更新和定价数据获取功能添加代理支持。

主要变更：
- 新增 update.proxy_url 配置项，支持 http/https/socks5/socks5h 协议
- 修改 GitHubReleaseClient 和 PricingRemoteClient 支持代理配置
- 更新 Wire 依赖注入，通过 Provider 函数传递配置
- 更新 Docker 配置文件，支持通过 UPDATE_PROXY_URL 环境变量设置代理

配置示例：
  update:
    proxy_url: "http://127.0.0.1:7890"

Docker 环境变量：
  UPDATE_PROXY_URL=http://host.docker.internal:7890

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/repository/github_release_service_test.go

@@ -39,8 +39,8 @@ func (t *testTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 
 func newTestGitHubReleaseClient() *githubReleaseClient {
 	return &githubReleaseClient{
-		httpClient:        &http.Client{},
-		allowPrivateHosts: true,
+		httpClient:         &http.Client{},
+		downloadHTTPClient: &http.Client{},
 	}
 }
 
@@ -234,7 +234,7 @@ func (s *GitHubReleaseServiceSuite) TestFetchLatestRelease_Success() {
 		httpClient: &http.Client{
 			Transport: &testTransport{testServerURL: s.srv.URL},
 		},
-		allowPrivateHosts: true,
+		downloadHTTPClient: &http.Client{},
 	}
 
 	release, err := s.client.FetchLatestRelease(context.Background(), "test/repo")
@@ -254,7 +254,7 @@ func (s *GitHubReleaseServiceSuite) TestFetchLatestRelease_Non200() {
 		httpClient: &http.Client{
 			Transport: &testTransport{testServerURL: s.srv.URL},
 		},
-		allowPrivateHosts: true,
+		downloadHTTPClient: &http.Client{},
 	}
 
 	_, err := s.client.FetchLatestRelease(context.Background(), "test/repo")
@@ -272,7 +272,7 @@ func (s *GitHubReleaseServiceSuite) TestFetchLatestRelease_InvalidJSON() {
 		httpClient: &http.Client{
 			Transport: &testTransport{testServerURL: s.srv.URL},
 		},
-		allowPrivateHosts: true,
+		downloadHTTPClient: &http.Client{},
 	}
 
 	_, err := s.client.FetchLatestRelease(context.Background(), "test/repo")
@@ -288,7 +288,7 @@ func (s *GitHubReleaseServiceSuite) TestFetchLatestRelease_ContextCancel() {
 		httpClient: &http.Client{
 			Transport: &testTransport{testServerURL: s.srv.URL},
 		},
-		allowPrivateHosts: true,
+		downloadHTTPClient: &http.Client{},
 	}
 
 	ctx, cancel := context.WithCancel(context.Background())