fix(beszel): prevent live subscription memory growth (#6448)

* fix(beszel): bound live stats subscriptions

* fix(beszel): avoid blocking subscription teardown

* fix(import): map new clock options

packages/api/src/router/widgets/bounded-async-queue.ts

@@ -0,0 +1,88 @@
+type QueueResult<T> = IteratorResult<T, undefined>;
+
+type PendingRead<T> = {
+  resolve: (result: QueueResult<T>) => void;
+  reject: (reason?: unknown) => void;
+};
+
+/**
+ * A small async queue for live streams.
+ *
+ * Live metrics are snapshots, so retaining every snapshot while a client is
+ * slow is both unnecessary and dangerous. When the queue is full, discard
+ * the oldest pending snapshot and keep the newest one.
+ */
+export class BoundedAsyncQueue<T> implements AsyncIterableIterator<T> {
+  private readonly values: T[] = [];
+  private pendingRead: PendingRead<T> | undefined;
+  private closed = false;
+  private failed = false;
+  private failure: unknown;
+
+  public constructor(private readonly maxSize: number) {
+    if (!Number.isInteger(maxSize) || maxSize < 1) {
+      throw new Error("A bounded async queue requires a positive integer max size");
+    }
+  }
+
+  public push(value: T): void {
+    if (this.closed) return;
+
+    if (this.pendingRead) {
+      const pendingRead = this.pendingRead;
+      this.pendingRead = undefined;
+      pendingRead.resolve({ value, done: false });
+      return;
+    }
+
+    if (this.values.length >= this.maxSize) this.values.shift();
+    this.values.push(value);
+  }
+
+  public close(): void {
+    if (this.closed) return;
+    this.closed = true;
+
+    if (this.pendingRead) {
+      const pendingRead = this.pendingRead;
+      this.pendingRead = undefined;
+      pendingRead.resolve({ value: undefined, done: true });
+    }
+  }
+
+  public fail(error: unknown): void {
+    if (this.closed) return;
+    this.closed = true;
+    this.failed = true;
+    this.failure = error;
+    this.values.length = 0;
+
+    if (this.pendingRead) {
+      const pendingRead = this.pendingRead;
+      this.pendingRead = undefined;
+      pendingRead.reject(error);
+    }
+  }
+
+  public next(): Promise<QueueResult<T>> {
+    if (this.values.length > 0) {
+      return Promise.resolve({ value: this.values.shift() as T, done: false });
+    }
+    if (this.failed) return Promise.reject(this.failure);
+    if (this.closed) return Promise.resolve({ value: undefined, done: true });
+
+    return new Promise<QueueResult<T>>((resolve, reject) => {
+      this.pendingRead = { resolve, reject };
+    });
+  }
+
+  public return(): Promise<QueueResult<T>> {
+    this.values.length = 0;
+    this.close();
+    return Promise.resolve({ value: undefined, done: true });
+  }
+
+  public [Symbol.asyncIterator](): AsyncIterableIterator<T> {
+    return this;
+  }
+}