feat(gateway): Cache-Driven RPM Buffer

- buffer 公式从 baseRPM/5 改为 concurrency + maxSessions
  保留 baseRPM/5 作为 floor 向后兼容
- 粘性路径 fallback 新增 [StickyCacheMiss] 结构化日志
  reason: rpm_red / gate_check / session_limit / wait_queue_full / account_cleared
- session_limit 路径跳过 wait queue 重试（RegisterSession 拒绝无副作用）
- 典型配置 buffer 从 3 提升至 13，大幅减少高峰期 Prompt Cache Miss

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/service/gateway_service.go

@@ -1418,19 +1418,24 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 				if containsInt64(routingAccountIDs, stickyAccountID) && !isExcluded(stickyAccountID) {
 					// 粘性账号在路由列表中，优先使用
 					if stickyAccount, ok := accountByID[stickyAccountID]; ok {
-						if s.isAccountSchedulableForSelection(stickyAccount) &&
+						var stickyCacheMissReason string
+
+						gatePass := s.isAccountSchedulableForSelection(stickyAccount) &&
 							s.isAccountAllowedForPlatform(stickyAccount, platform, useMixed) &&
 							(requestedModel == "" || s.isModelSupportedByAccountWithContext(ctx, stickyAccount, requestedModel)) &&
 							s.isAccountSchedulableForModelSelection(ctx, stickyAccount, requestedModel) &&
 							s.isAccountSchedulableForQuota(stickyAccount) &&
-							s.isAccountSchedulableForWindowCost(ctx, stickyAccount, true) &&
+							s.isAccountSchedulableForWindowCost(ctx, stickyAccount, true)
 
-							s.isAccountSchedulableForRPM(ctx, stickyAccount, true) { // 粘性会话窗口费用+RPM 检查
+						rpmPass := gatePass && s.isAccountSchedulableForRPM(ctx, stickyAccount, true)
+
+						if rpmPass { // 粘性会话窗口费用+RPM 检查
 							result, err := s.tryAcquireAccountSlot(ctx, stickyAccountID, stickyAccount.Concurrency)
 							if err == nil && result.Acquired {
 								// 会话数量限制检查
 								if !s.checkAndRegisterSession(ctx, stickyAccount, sessionHash) {
 									result.ReleaseFunc() // 释放槽位
+									stickyCacheMissReason = "session_limit"
 									// 继续到负载感知选择
 								} else {
 									if s.debugModelRoutingEnabled() {
@@ -1444,27 +1449,49 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 								}
 							}
 
-							waitingCount, _ := s.concurrencyService.GetAccountWaitingCount(ctx, stickyAccountID)
-							if waitingCount < cfg.StickySessionMaxWaiting {
-								// 会话数量限制检查（等待计划也需要占用会话配额）
-								if !s.checkAndRegisterSession(ctx, stickyAccount, sessionHash) {
-									// 会话限制已满，继续到负载感知选择
+							if stickyCacheMissReason == "" {
+								waitingCount, _ := s.concurrencyService.GetAccountWaitingCount(ctx, stickyAccountID)
+								if waitingCount < cfg.StickySessionMaxWaiting {
+									// 会话数量限制检查（等待计划也需要占用会话配额）
+									if !s.checkAndRegisterSession(ctx, stickyAccount, sessionHash) {
+										stickyCacheMissReason = "session_limit"
+										// 会话限制已满，继续到负载感知选择
+									} else {
+										return &AccountSelectionResult{
+											Account: stickyAccount,
+											WaitPlan: &AccountWaitPlan{
+												AccountID:      stickyAccountID,
+												MaxConcurrency: stickyAccount.Concurrency,
+												Timeout:        cfg.StickySessionWaitTimeout,
+												MaxWaiting:     cfg.StickySessionMaxWaiting,
+											},
+										}, nil
+									}
 								} else {
-									return &AccountSelectionResult{
-										Account: stickyAccount,
-										WaitPlan: &AccountWaitPlan{
-											AccountID:      stickyAccountID,
-											MaxConcurrency: stickyAccount.Concurrency,
-											Timeout:        cfg.StickySessionWaitTimeout,
-											MaxWaiting:     cfg.StickySessionMaxWaiting,
-										},
-									}, nil
+									stickyCacheMissReason = "wait_queue_full"
 								}
 							}
 							// 粘性账号槽位满且等待队列已满，继续使用负载感知选择
+						} else if !gatePass {
+							stickyCacheMissReason = "gate_check"
+						} else {
+							stickyCacheMissReason = "rpm_red"
+						}
+
+						// 记录粘性缓存未命中的结构化日志
+						if stickyCacheMissReason != "" {
+							baseRPM := stickyAccount.GetBaseRPM()
+							var currentRPM int
+							if count, ok := rpmFromPrefetchContext(ctx, stickyAccount.ID); ok {
+								currentRPM = count
+							}
+							logger.LegacyPrintf("service.gateway", "[StickyCacheMiss] reason=%s account_id=%d session=%s current_rpm=%d base_rpm=%d",
+								stickyCacheMissReason, stickyAccountID, shortSessionHash(sessionHash), currentRPM, baseRPM)
 						}
 					} else {
 						_ = s.cache.DeleteSessionAccountID(ctx, derefGroupID(groupID), sessionHash)
+						logger.LegacyPrintf("service.gateway", "[StickyCacheMiss] reason=account_cleared account_id=%d session=%s current_rpm=0 base_rpm=0",
+							stickyAccountID, shortSessionHash(sessionHash))
 					}
 				}
 			}