feat(smart-proxy): add background concurrent certificate provisioning with per-domain timeouts and concurrency control

ts/proxies/smart-proxy/utils/concurrency-semaphore.ts

@@ -0,0 +1,28 @@
+/**
+ * Async concurrency semaphore — limits the number of concurrent async operations.
+ */
+export class ConcurrencySemaphore {
+  private running = 0;
+  private waitQueue: Array<() => void> = [];
+
+  constructor(private readonly maxConcurrency: number) {}
+
+  async acquire(): Promise<void> {
+    if (this.running < this.maxConcurrency) {
+      this.running++;
+      return;
+    }
+    return new Promise<void>((resolve) => {
+      this.waitQueue.push(() => {
+        this.running++;
+        resolve();
+      });
+    });
+  }
+
+  release(): void {
+    this.running--;
+    const next = this.waitQueue.shift();
+    if (next) next();
+  }
+}