feat: add partial index on client_applications annouced FALSE (#6166)

We have customers with tens or hundreds of thousands of applications,
and we have a scheduler running that sets application fields to
`announced` as true. However, every time it runs, it queries the entire
table, which is slow and causes database connection acquisition issues.
To make it faster, we added a partial index to the table.

src/migrations/20240207164033-client-applications-announced-index.js

@@ -0,0 +1,17 @@
+'use strict';
+
+exports.up = function(db, cb) {
+    db.runSql(
+        `
+            CREATE INDEX IF NOT EXISTS idx_client_applications_announced_false ON client_applications (announced)
+                WHERE announced = FALSE;
+        `,
+        cb,
+    );
+};
+
+exports.down = function(db, cb) {
+    db.runSql(`
+        DROP INDEX IF EXISTS idx_client_applications_announced_false;
+    `, cb);
+};