fix(login-perf): kill relay REQ storms and feedback loops

Real-environment console logs revealed nip-07 login pegging the CPU and
starving encrypted-list decryption via relay REQ storms (~1000 REQ/s):

- moderationService.rebuildTrustedContacts emitted "contacts"
  unconditionally, forming an emit->fetch->emit loop (~1600 kind-3 REQ/s).
  Now emits only on actual change. Regression test added.
- Per-video live report (kind 1984) and view-count (kind 30079)
  subscriptions fan out O(videos x relays) and re-fire on every flaky-relay
  reconnect. Circuit-broken behind flags until batched (see refactor plan).
- onVideosShouldRefresh now coalesces forced reloads (2s leading-edge
  throttle) so list-change signals can't drive a reload loop.
- nip-07 queue reserves one foreground slot; DM decryption demoted to LOW
  so background DM backfill can't starve feed-driving lists. Test added.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

tests/moderation/contacts-emit-on-change.test.mjs

@@ -0,0 +1,54 @@
+// Regression test for the nip-07-login CPU-peg / relay REQ storm.
+//
+// Scenario (SCN-contacts-emit-on-change):
+//   Given the UI re-fetches trusted contacts whenever the moderation service
+//     emits "contacts" (profileModalController.populateFriendsList ->
+//     ensureViewerContactsLoaded -> fetchTrustedContacts -> rebuildTrustedContacts),
+//   When rebuildTrustedContacts is invoked with an UNCHANGED contact set,
+//   Then "contacts" is NOT emitted — otherwise the emit -> fetch -> emit chain
+//     spins the CPU and floods relays with kind-3 REQs (observed ~1600/s).
+//   And when the set genuinely changes, exactly one "contacts" event fires.
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import {
+  withMockedNostrTools,
+  createModerationServiceHarness,
+} from "../helpers/moderation-test-helpers.mjs";
+
+const A = "a".repeat(64);
+const B = "b".repeat(64);
+
+test("rebuildTrustedContacts emits 'contacts' only when the set changes", (t) => {
+  withMockedNostrTools(t);
+  const { service, capture } = createModerationServiceHarness(t);
+  const emits = capture("contacts");
+
+  // Establish a baseline population (a real change from the initial state).
+  service.rebuildTrustedContacts(new Set([A, B]));
+  const afterFirst = emits.length;
+  assert.ok(afterFirst >= 1, "first real population should emit at least once");
+
+  // Re-applying the SAME set repeatedly must NOT emit. This is the exact loop
+  // that pegged the CPU: identical empty/unchanged rebuilds re-emitting forever.
+  service.rebuildTrustedContacts(new Set([A, B]));
+  service.rebuildTrustedContacts(new Set([A, B]));
+  service.rebuildTrustedContacts(new Set([A, B]));
+  assert.equal(
+    emits.length,
+    afterFirst,
+    "unchanged contact sets must not emit (loop guard)",
+  );
+
+  // A genuine change emits exactly once more.
+  service.rebuildTrustedContacts(new Set([A]));
+  assert.equal(emits.length, afterFirst + 1, "a changed set emits once");
+
+  // ...and re-applying that changed set is again silent.
+  service.rebuildTrustedContacts(new Set([A]));
+  assert.equal(
+    emits.length,
+    afterFirst + 1,
+    "repeating the changed set must not emit",
+  );
+});