fix: 修复代码审核发现的10个问题（P0安全+P1数据一致性+P2性能优化）

P0: OpenAI SSE 错误消息 JSON 注入 — 使用 json.Marshal 替代 fmt.Sprintf
P1: subscription 续期包裹 Ent 事务确保原子性
P1: CSP nonce 生成处理 crypto/rand 错误，失败降级为 unsafe-inline
P1: singleflight 透传数据库真实错误，不再吞没为 not found
P1: GetUserSubscriptionsWithProgress 提取 calculateProgress 消除 N+1
P2: billing_cache/gateway_helper 迁移到 math/rand/v2 消除全局锁争用
P2: generateRandomID 降级分支增加原子计数器防碰撞
P2: CORS 非白名单 origin 不再设置 Allow-Headers/Methods/Max-Age
P2: Turnstile 验证移除 VerifyCode 空值跳过条件防绕过
P2: Redis Cluster Lua 脚本空 KEYS 添加兼容性警告注释

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/handler/auth_handler.go

@@ -113,12 +113,11 @@ func (h *AuthHandler) Register(c *gin.Context) {
 		return
 	}
 
-	// Turnstile 验证（当提供了邮箱验证码时跳过，因为发送验证码时已验证过）
-	if req.VerifyCode == "" {
-		if err := h.authService.VerifyTurnstile(c.Request.Context(), req.TurnstileToken, ip.GetClientIP(c)); err != nil {
-			response.ErrorFrom(c, err)
-			return
-		}
+	// Turnstile 验证 — 始终执行，防止绕过
+	// TODO: 确认前端在提交邮箱验证码注册时也传递了 turnstile_token
+	if err := h.authService.VerifyTurnstile(c.Request.Context(), req.TurnstileToken, ip.GetClientIP(c)); err != nil {
+		response.ErrorFrom(c, err)
+		return
 	}
 
 	_, user, err := h.authService.RegisterWithVerification(c.Request.Context(), req.Email, req.Password, req.VerifyCode, req.PromoCode, req.InvitationCode)

backend/internal/handler/gateway_helper.go

@@ -4,7 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"math/rand"
+	"math/rand/v2"
 	"net/http"
 	"sync"
 	"time"
@@ -242,7 +242,6 @@ func (h *ConcurrencyHelper) waitForSlotWithPingTimeout(c *gin.Context, slotType
 	backoff := initialBackoff
 	timer := time.NewTimer(backoff)
 	defer timer.Stop()
-	rng := rand.New(rand.NewSource(time.Now().UnixNano()))
 
 	for {
 		select {
@@ -284,7 +283,7 @@ func (h *ConcurrencyHelper) waitForSlotWithPingTimeout(c *gin.Context, slotType
 			if result.Acquired {
 				return result.ReleaseFunc, nil
 			}
-			backoff = nextBackoff(backoff, rng)
+			backoff = nextBackoff(backoff)
 			timer.Reset(backoff)
 		}
 	}
@@ -298,20 +297,16 @@ func (h *ConcurrencyHelper) AcquireAccountSlotWithWaitTimeout(c *gin.Context, ac
 // nextBackoff 计算下一次退避时间
 // 性能优化：使用指数退避 + 随机抖动，避免惊群效应
 // current: 当前退避时间
-// rng: 随机数生成器（可为 nil，此时不添加抖动）
 // 返回值：下一次退避时间（100ms ~ 2s 之间）
-func nextBackoff(current time.Duration, rng *rand.Rand) time.Duration {
+func nextBackoff(current time.Duration) time.Duration {
 	// 指数退避：当前时间 * 1.5
 	next := time.Duration(float64(current) * backoffMultiplier)
 	if next > maxBackoff {
 		next = maxBackoff
 	}
-	if rng == nil {
-		return next
-	}
 	// 添加 ±20% 的随机抖动（jitter 范围 0.8 ~ 1.2）
 	// 抖动可以分散多个请求的重试时间点，避免同时冲击 Redis
-	jitter := 0.8 + rng.Float64()*0.4
+	jitter := 0.8 + rand.Float64()*0.4
 	jittered := time.Duration(float64(next) * jitter)
 	if jittered < initialBackoff {
 		return initialBackoff

backend/internal/handler/openai_gateway_handler.go

@@ -396,8 +396,19 @@ func (h *OpenAIGatewayHandler) handleStreamingAwareError(c *gin.Context, status
 		// Stream already started, send error as SSE event then close
 		flusher, ok := c.Writer.(http.Flusher)
 		if ok {
-			// Send error event in OpenAI SSE format
-			errorEvent := fmt.Sprintf(`event: error`+"\n"+`data: {"error": {"type": "%s", "message": "%s"}}`+"\n\n", errType, message)
+			// Send error event in OpenAI SSE format with proper JSON marshaling
+			errorData := map[string]any{
+				"error": map[string]string{
+					"type":    errType,
+					"message": message,
+				},
+			}
+			jsonBytes, err := json.Marshal(errorData)
+			if err != nil {
+				_ = c.Error(err)
+				return
+			}
+			errorEvent := fmt.Sprintf("event: error\ndata: %s\n\n", string(jsonBytes))
 			if _, err := fmt.Fprint(c.Writer, errorEvent); err != nil {
 				_ = c.Error(err)
 			}