feat: complete TASK-WP0-02 contract baseline

apps/api/src/event-hub.ts

@@ -0,0 +1,31 @@
+import { isSseEvent, type SseEvent } from "@dada/shared-contracts";
+
+interface Connection {
+  close: () => void;
+  send: (event: SseEvent) => void;
+}
+
+export class EventHub {
+  readonly #connections = new Set<Connection>();
+
+  get subscriberCount() {
+    return this.#connections.size;
+  }
+
+  connect(send: Connection["send"], close: Connection["close"]) {
+    const connection = { close, send };
+    this.#connections.add(connection);
+    return () => this.#connections.delete(connection);
+  }
+
+  publish(event: SseEvent) {
+    if (!isSseEvent(event)) throw new Error("SSE event does not match the frozen non-sensitive schema.");
+    for (const connection of this.#connections) connection.send(event);
+  }
+
+  disconnectAll() {
+    const connections = [...this.#connections];
+    this.#connections.clear();
+    for (const connection of connections) connection.close();
+  }
+}