fix(subsUnion): move geo-union math to a worker thread, not just yields

The previous fix (yielding to the event loop between turf.union calls)
was not enough: once the merged polygon gets complex with thousands of
subscriptions, a single turf.union call can itself take seconds, and
yielding between iterations doesn't help when one iteration alone
blocks that long. Confirmed in staging: the site was still fully
unresponsive (Cloudflare 524, curl hanging 2+ minutes, healthcheck
failing) while a recompute ran.

Validation/decoration (addNoisy/noNoisy, cheap) stays on the main
thread; the actual circle+union chain now runs in a worker_thread
(private/workers/unionWorker.js, plain CommonJS so meteor build copies
it verbatim instead of compiling it) so the main event loop serving
DDP/HTTP is never blocked by it, regardless of how slow any single
turf call gets.

private/workers/unionWorker.js

@@ -0,0 +1,40 @@
+// Plain CommonJS worker_threads entry point, NOT part of the Meteor imports
+// graph (lives under private/ so `meteor build` copies it verbatim into
+// programs/server/assets/app/workers/unionWorker.js instead of compiling it).
+//
+// Runs the CPU-heavy part of the subscriptions geo-union (circle generation +
+// sequential turf.union chain) off the main thread. A single tunion() call
+// against an already-complex merged polygon can itself take seconds once
+// there are thousands of subscriptions — yielding between iterations on the
+// main thread doesn't help when one iteration alone blocks that long, so this
+// needs a real worker thread, not just cooperative scheduling.
+//
+// workerData.subs must already be validated (location.lat/lon/distance
+// present) and decorated (addNoisy/noNoisy already applied) by the caller.
+const { parentPort, workerData } = require('worker_threads');
+const tcircle = require('@turf/circle').default;
+const tunion = require('@turf/union').default;
+const ttrunc = require('@turf/truncate').default;
+
+const truncOptions = { precision: 6, coordinates: 2 };
+
+const run = () => {
+  const { subs } = workerData;
+  const unionGroup = subs.map(s => tcircle(
+    [s.location.lon, s.location.lat],
+    s.distance,
+    { units: 'kilometers', steps: 144 }
+  ));
+
+  let unionTemp = unionGroup.length > 0 ? ttrunc(unionGroup[0], truncOptions) : null;
+  for (let i = 1; i < unionGroup.length; i += 1) {
+    unionTemp = ttrunc(tunion(unionTemp, unionGroup[i]), truncOptions);
+  }
+  return unionTemp;
+};
+
+try {
+  parentPort.postMessage({ union: run() });
+} catch (e) {
+  parentPort.postMessage({ error: e.message });
+}