fix(流式): 提升SSE稳定性并统一超时配置

- 扩展SSE行长与间隔超时处理，补充keepalive

- 写入失败与超长行时发送错误事件，修复并发释放

- 同步默认配置与示例配置，更新Caddy超时/压缩规则

- 新增OpenAI流式超时与超长行测试

测试: go test ./...

backend/internal/handler/gateway_handler.go

@@ -11,6 +11,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/antigravity"
 	pkgerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/claude"
@@ -39,14 +40,19 @@ func NewGatewayHandler(
 	userService *service.UserService,
 	concurrencyService *service.ConcurrencyService,
 	billingCacheService *service.BillingCacheService,
+	cfg *config.Config,
 ) *GatewayHandler {
+	pingInterval := time.Duration(0)
+	if cfg != nil {
+		pingInterval = time.Duration(cfg.Concurrency.PingInterval) * time.Second
+	}
 	return &GatewayHandler{
 		gatewayService:            gatewayService,
 		geminiCompatService:       geminiCompatService,
 		antigravityGatewayService: antigravityGatewayService,
 		userService:               userService,
 		billingCacheService:       billingCacheService,
-		concurrencyHelper:         NewConcurrencyHelper(concurrencyService, SSEPingFormatClaude),
+		concurrencyHelper:         NewConcurrencyHelper(concurrencyService, SSEPingFormatClaude, pingInterval),
 	}
 }
 
@@ -122,6 +128,8 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 		h.handleConcurrencyError(c, err, "user", streamStarted)
 		return
 	}
+	// 在请求结束或 Context 取消时确保释放槽位，避免客户端断开造成泄漏
+	userReleaseFunc = wrapReleaseOnDone(c.Request.Context(), userReleaseFunc)
 	if userReleaseFunc != nil {
 		defer userReleaseFunc()
 	}
@@ -222,6 +230,9 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					log.Printf("Bind sticky session failed: %v", err)
 				}
 			}
+			// 账号槽位/等待计数需要在超时或断开时安全回收
+			accountReleaseFunc = wrapReleaseOnDone(c.Request.Context(), accountReleaseFunc)
+			accountWaitRelease = wrapReleaseOnDone(c.Request.Context(), accountWaitRelease)
 
 			// 转发请求 - 根据账号平台分流
 			var result *service.ForwardResult
@@ -346,6 +357,9 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 				log.Printf("Bind sticky session failed: %v", err)
 			}
 		}
+		// 账号槽位/等待计数需要在超时或断开时安全回收
+		accountReleaseFunc = wrapReleaseOnDone(c.Request.Context(), accountReleaseFunc)
+		accountWaitRelease = wrapReleaseOnDone(c.Request.Context(), accountWaitRelease)
 
 		// 转发请求 - 根据账号平台分流
 		var result *service.ForwardResult