perf: bound relay background concurrency to 3

Replaces unbounded `Promise.all(relays.map(...))` calls with `pMap` using a concurrency limit of 3 for background operations like history hydration, metadata fetching, and DM sending.
This prevents network saturation and potential main-thread jank when many relays are configured.
Includes benchmark verification script `benchmarks/relay_concurrency_repro.mjs`.
Audited upload docs and confirmed accuracy.

Co-authored-by: PR0M3TH3AN <[email protected]>

benchmarks/relay_concurrency_repro.mjs

@@ -0,0 +1,101 @@
+import { NostrClient } from "../js/nostr/client.js";
+import { pMap } from "../js/utils/asyncUtils.js";
+
+// Mock globals for Node environment
+globalThis.WebSocket = class MockWebSocket {};
+globalThis.localStorage = {
+  getItem: () => null,
+  setItem: () => {},
+  removeItem: () => {},
+  clear: () => {}
+};
+globalThis.indexedDB = {
+  open: () => ({
+    result: {
+      createObjectStore: () => {},
+      transaction: () => ({ objectStore: () => ({ get: () => {}, put: () => {} }) })
+    },
+    onupgradeneeded: () => {},
+    onsuccess: () => {}
+  })
+};
+
+// Mock dependencies
+class MockPool {
+  constructor() {
+    this.activeRequests = 0;
+    this.maxConcurrent = 0;
+  }
+
+  async list(relays, filters) {
+    this.activeRequests++;
+    if (this.activeRequests > this.maxConcurrent) {
+      this.maxConcurrent = this.activeRequests;
+    }
+
+    // Simulate network delay
+    await new Promise(resolve => setTimeout(resolve, 50));
+
+    this.activeRequests--;
+    return [];
+  }
+
+  async get(relays, filter) {
+    return null;
+  }
+}
+
+async function runBenchmark() {
+  console.log("Starting Relay Concurrency Benchmark...");
+
+  const client = new NostrClient();
+  const mockPool = new MockPool();
+  client.pool = mockPool;
+
+  // Simulate 20 relays
+  const relays = Array.from({ length: 20 }, (_, i) => `wss://relay${i}.example.com`);
+  client.relays = relays;
+  client.readRelays = relays;
+
+  // 1. Benchmark fetchAndCacheNip71Metadata
+  console.log("\nTesting fetchAndCacheNip71Metadata with 20 relays...");
+  mockPool.maxConcurrent = 0;
+
+  const pointerMap = new Map();
+  pointerMap.set("e:123", { type: "e", value: "123", relays: [] });
+
+  await client.fetchAndCacheNip71Metadata(pointerMap, ["e:123"]);
+
+  console.log(`Max concurrent requests: ${mockPool.maxConcurrent}`);
+  if (mockPool.maxConcurrent > 5) {
+    console.log("FAIL: Concurrency is unbounded!");
+  } else {
+    console.log("PASS: Concurrency is bounded.");
+  }
+
+  // 2. Benchmark hydrateVideoHistory
+  console.log("\nTesting hydrateVideoHistory with 20 relays...");
+  mockPool.maxConcurrent = 0;
+
+  const video = {
+    id: "v1",
+    videoRootId: "root1",
+    tags: [["d", "d1"]],
+    pubkey: "pub1"
+  };
+
+  // Prime local cache to force relay fetch
+  // We need to simulate that we found <= 1 version locally
+  client.allEvents.set("v1", video);
+
+  await client.hydrateVideoHistory(video);
+
+  console.log(`Max concurrent requests: ${mockPool.maxConcurrent}`);
+    if (mockPool.maxConcurrent > 5) {
+    console.log("FAIL: Concurrency is unbounded!");
+  } else {
+    console.log("PASS: Concurrency is bounded.");
+  }
+}
+
+runBenchmark().catch(console.error);