perf(relay): full-chain optimization — key pool, model sync, SSE stream

Phase 1 (Key Pool correctness):
- RPM: fixed-minute window → sliding 60s aggregation (prevents 2x burst)
- Remove fallback-to-provider-key bypass when all keys rate-limited
- SSE semaphore: 16→64 permits, cleanup delay 60s→5s
- Default 429 cooldown: 5min→60s (better for Coding Plan quotas)
- Expire old key_usage_window rows on record

Phase 2 (Frontend model sync):
- currentModel empty-string fallback to glm-4-flash-250414 in relay client
- Merge duplicate listModels() calls in connectionStore SaaS path
- Show ModelSelector in Tauri mode when models available
- Clear currentModel on SaaS logout

Phase 3 (Relay performance):
- Key Pool: DashMap in-memory cache (TTL 5s) for select_best_key
- Cache invalidation on 429 marking

Phase 4 (SSE stream):
- AbortController integration for user-cancelled streams
- SSE parsing: split by event boundaries (\n\n) instead of per-line
- streamStore cancelStream adapts to 0-arg and 1-arg cancel fns

crates/zclaw-saas/src/relay/service.rs

@@ -19,8 +19,8 @@ const STREAMBRIDGE_HEARTBEAT_INTERVAL: Duration = Duration::from_secs(15);
 /// 实测 Kimi for Coding 的 thinking→content 间隔可达 60s+，需要更宽容的超时。
 const STREAMBRIDGE_TIMEOUT: Duration = Duration::from_secs(180);
 
-/// 流结束后延迟清理的时间窗口
-const STREAMBRIDGE_CLEANUP_DELAY: Duration = Duration::from_secs(60);
+/// 流结束后延迟清理的时间窗口（缩短到 5s，仅用于 Arc 引用释放）
+const STREAMBRIDGE_CLEANUP_DELAY: Duration = Duration::from_secs(5);
 
 /// 判断 HTTP 状态码是否为可重试的瞬态错误 (5xx + 429)
 fn is_retryable_status(status: u16) -> bool {
@@ -357,7 +357,7 @@ pub async fn execute_relay(
                     // SSE 流结束后异步记录 usage + Key 使用量
                     // 使用全局 Arc<Semaphore> 限制并发 spawned tasks，防止高并发时耗尽连接池
                     static SSE_SPAWN_SEMAPHORE: std::sync::OnceLock<Arc<tokio::sync::Semaphore>> = std::sync::OnceLock::new();
-                    let semaphore = SSE_SPAWN_SEMAPHORE.get_or_init(|| Arc::new(tokio::sync::Semaphore::new(16)));
+                    let semaphore = SSE_SPAWN_SEMAPHORE.get_or_init(|| Arc::new(tokio::sync::Semaphore::new(64)));
                     let permit = match semaphore.clone().try_acquire_owned() {
                         Ok(p) => p,
                         Err(_) => {