chore: limit the amount of unannounced events we announce (#4845)

## About the changes
When the events table is large we might be doing a full table scan
searching for unannounced events. We spotted it due to a performance
alert and confirmed in AWS performance insights

![image](https://github.com/Unleash/unleash/assets/455064/8e815fa3-7a1b-4453-881a-98a148eae119)

The proposal is to limit this operation to 500 events (rule of thumb)
per round
f82ae354eb/src/lib/services/index.ts (L141-L147)
and also ignore the events older than a day (because it seems
reasonable)


## Discussion points
**Idea**: split the `events` table into `recent_events` and
`historical_events`. Recent can be anything from a day/week/month. This
would help with recurrent queries that rely on recent data from the
event's table such as optimal 304 calculation or event this scheduled
task that sends unannounced events.

src/lib/db/event-store.test.ts

@@ -1,6 +1,8 @@
 import knex from 'knex';
 import EventStore from './event-store';
 import getLogger from '../../test/fixtures/no-logger';
+import { subHours, formatRFC3339 } from 'date-fns';
+import dbInit from '../../test/e2e/helpers/database-init';
 
 beforeAll(() => {
     getLogger.setMuteError(true);
@@ -28,3 +30,38 @@ test('Trying to get events by name if db fails should yield empty list', async (
     expect(events).toBeTruthy();
     expect(events.length).toBe(0);
 });
+
+test.each([
+    {
+        createdAt: formatRFC3339(subHours(new Date(), 1)),
+        expectedCount: 1,
+    },
+    {
+        createdAt: formatRFC3339(subHours(new Date(), 23)),
+        expectedCount: 1,
+    },
+    {
+        createdAt: formatRFC3339(subHours(new Date(), 25)),
+        expectedCount: 0,
+    },
+])(
+    'Find unnanounced events is capped to last 24hs',
+    async ({ createdAt, expectedCount }) => {
+        const db = await dbInit('events_test', getLogger);
+        const type = 'application-created' as const;
+        const insertQuery = db.rawDatabase('events');
+        await insertQuery
+            .insert({
+                type,
+                created_at: createdAt,
+                created_by: 'a test',
+                data: { name: 'test', createdAt },
+            })
+            .returning(['id']);
+
+        const store = new EventStore(db.rawDatabase, getLogger);
+        const events = await store.setUnannouncedToAnnounced();
+        expect(events).toBeTruthy();
+        expect(events.length).toBe(expectedCount);
+    },
+);

src/lib/db/event-store.ts

@@ -12,6 +12,7 @@ import { sharedEventEmitter } from '../util/anyEventEmitter';
 import { Db } from './db';
 import { Knex } from 'knex';
 import EventEmitter from 'events';
+import { subDays } from 'date-fns';
 
 const EVENT_COLUMNS = [
     'id',
@@ -407,12 +408,14 @@ class EventStore implements IEventStore {
         return this.eventEmitter.off(eventName, listener);
     }
 
-    private async setUnannouncedToAnnounced(): Promise<IEvent[]> {
+    async setUnannouncedToAnnounced(): Promise<IEvent[]> {
         const rows = await this.db(TABLE)
             .update({ announced: true })
             .where('announced', false)
             .whereNotNull('announced')
-            .returning(EVENT_COLUMNS);
+            .where('created_at', '>', subDays(Date.now(), 1))
+            .returning(EVENT_COLUMNS)
+            .limit(500);
 
         return rows.map(this.rowToEvent);
     }