Replace konecty:mongo-counter with inline implementation

Remove the konecty:mongo-counter Meteor package dependency and replace
it with a minimal inline incrementCounter function using raw MongoDB
findOneAndUpdate. Also exports an async version for future Meteor 3.0
migration. No data migration needed — uses the same next_val field.

models/counters.js

@@ -1 +1,25 @@
+import { Meteor } from 'meteor/meteor';
+
 Counters = new Mongo.Collection('counters');
+
+// Async version (for future Meteor 3.0 migration)
+async function incrementCounterAsync(counterName, amount = 1) {
+  const result = await Counters.rawCollection().findOneAndUpdate(
+    { _id: counterName },
+    { $inc: { next_val: amount } },
+    { upsert: true, returnDocument: 'after' },
+  );
+  return result.value ? result.value.next_val : result.next_val;
+}
+
+// Sync version (for Meteor 2.x autoValue compatibility)
+const incrementCounter = Meteor.wrapAsync(async (counterName, amount, callback) => {
+  try {
+    const result = await incrementCounterAsync(counterName, amount);
+    callback(null, result);
+  } catch (err) {
+    callback(err);
+  }
+});
+
+export { incrementCounter, incrementCounterAsync };