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.

imports/startup/server/subsUnion.js

@@ -37,7 +37,20 @@ Meteor.startup(async () => {
 
   const process = async (isPublic) => {
     const subscribers = await Subscriptions.find().fetchAsync();
-    const union = await calcUnionAsync(subscribers, isPublic ? addNoisy : noNoisy);
+    const decorate = isPublic ? addNoisy : noNoisy;
+    const validSubs = subscribers.filter((osub) => {
+      const valid = osub.location && osub.location.lat && osub.location.lon && osub.distance;
+      if (!valid) console.info(`Wrong element to do union ${JSON.stringify(osub)}`);
+      return valid;
+    });
+    const decorated = validSubs.map(decorate);
+
+    let union = null;
+    try {
+      union = await calcUnionAsync(decorated);
+    } catch (e) {
+      console.error('subsUnion worker failed', e);
+    }
     const bounds = union === null ? null : L.geoJSON(union).getBounds();
 
     const publicl = isPublic ? 'public' : 'private';