feat(proxy): 集中代理 URL 验证并实现全局 fail-fast

提取 proxyurl.Parse() 公共包，将分散在 6 处的代理 URL 验证逻辑
统一收敛，确保无效代理配置在创建时立即失败，永不静默回退直连。

主要变更：
- 新增 proxyurl 包：统一 TrimSpace → url.Parse → Host 校验 → Scheme 白名单
- socks5:// 自动升级为 socks5h://，防止 DNS 泄漏（大小写不敏感）
- antigravity: http.ProxyURL → proxyutil.ConfigureTransportProxy 支持 SOCKS5
- openai_oauth: 删除 newOpenAIOAuthHTTPClient，收编至 httpclient.GetClient
- 移除未使用的 ProxyStrict 字段（fail-fast 已是全局默认行为）
- 补充 15 个 proxyurl 测试 + pricing/usage fail-fast 测试

backend/internal/repository/github_release_service.go

@@ -5,8 +5,10 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"log/slog"
 	"net/http"
 	"os"
+	"strings"
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/pkg/httpclient"
@@ -24,13 +26,19 @@ type githubReleaseClientError struct {
 
 // NewGitHubReleaseClient 创建 GitHub Release 客户端
 // proxyURL 为空时直连 GitHub，支持 http/https/socks5/socks5h 协议
+// 代理配置失败时行为由 allowDirectOnProxyError 控制：
+//   - false（默认）：返回错误占位客户端，禁止回退到直连
+//   - true：回退到直连（仅限管理员显式开启）
 func NewGitHubReleaseClient(proxyURL string, allowDirectOnProxyError bool) service.GitHubReleaseClient {
+	// 安全说明：httpclient.GetClient 的错误链（url.Parse / proxyutil）不含明文代理凭据，
+	// 但仍通过 slog 仅在服务端日志记录，不会暴露给 HTTP 响应。
 	sharedClient, err := httpclient.GetClient(httpclient.Options{
 		Timeout:  30 * time.Second,
 		ProxyURL: proxyURL,
 	})
 	if err != nil {
-		if proxyURL != "" && !allowDirectOnProxyError {
+		if strings.TrimSpace(proxyURL) != "" && !allowDirectOnProxyError {
+			slog.Warn("proxy client init failed, all requests will fail", "service", "github_release", "error", err)
 			return &githubReleaseClientError{err: fmt.Errorf("proxy client init failed and direct fallback is disabled; set security.proxy_fallback.allow_direct_on_error=true to allow fallback: %w", err)}
 		}
 		sharedClient = &http.Client{Timeout: 30 * time.Second}
@@ -42,7 +50,8 @@ func NewGitHubReleaseClient(proxyURL string, allowDirectOnProxyError bool) servi
 		ProxyURL: proxyURL,
 	})
 	if err != nil {
-		if proxyURL != "" && !allowDirectOnProxyError {
+		if strings.TrimSpace(proxyURL) != "" && !allowDirectOnProxyError {
+			slog.Warn("proxy download client init failed, all requests will fail", "service", "github_release", "error", err)
 			return &githubReleaseClientError{err: fmt.Errorf("proxy client init failed and direct fallback is disabled; set security.proxy_fallback.allow_direct_on_error=true to allow fallback: %w", err)}
 		}
 		downloadClient = &http.Client{Timeout: 10 * time.Minute}