按发生时间倒序展示客户端埋点

调整事件查询排序及分页游标，保留入库时间快照隔离。
同步后台说明、定向测试和技术文档。

scripts/agc-analytics-smoke.mjs

@@ -160,6 +160,7 @@ async function verify(url, serviceToken, outsiderToken) {
       ),
     );
   const first = batch([event(), event(), event()]);
+  first.events[0].event_time = '2026-09-21T12:01:00.123Z';
   assert.deepEqual(ok(await upload(first)), {
     acknowledged_batch_ids: [first.batch_id],
     event_count: 3,
@@ -207,6 +208,7 @@ async function verify(url, serviceToken, outsiderToken) {
   const page1 = await list({ limit: 2 });
   assert(page1.nextCursor);
   const late = batch([event()]);
+  late.events[0].event_time = '2026-09-21T11:00:00.123Z';
   ok(await upload(late));
   const seen = [...page1.entries];
   let cursor = page1.nextCursor;
@@ -225,17 +227,18 @@ async function verify(url, serviceToken, outsiderToken) {
     const previous = seen[index - 1];
     const current = seen[index];
     assert(
-      previous.receivedAt > current.receivedAt ||
-        (previous.receivedAt === current.receivedAt &&
+      previous.eventTime > current.eventTime ||
+        (previous.eventTime === current.eventTime &&
           previous.eventId > current.eventId),
-      'Stable receipt/event ID descending order',
+      'Stable event time/event ID descending order',
     );
   }
   const refreshed = await list();
   assert.equal(refreshed.entries.length, 5);
-  assert.equal(refreshed.entries[0].eventId, late.events[0].event_id);
+  assert.equal(refreshed.entries[0].eventId, first.events[0].event_id);
+  assert.equal(refreshed.entries.at(-1).eventId, late.events[0].event_id);
   assert.equal(refreshed.entries[0].projectId, null);
-  assert.equal(refreshed.entries[0].eventTime, '2026-09-21T12:00:00.123Z');
+  assert.equal(refreshed.entries[0].eventTime, '2026-09-21T12:01:00.123Z');
   for (const [name, input] of [
     ['upload_agc_analytics_batch', first],
     ['list_agc_tracking_events', {}],