chore: AI chats db migration (#8460)

https://linear.app/unleash/issue/2-2845/db-migration-ai-chats

Adds a new DB migration for creating a new `ai_chats` table, used on the
Unleash AI experiment.

src/migrations/20241016090534-ai-chats.js

@@ -0,0 +1,26 @@
+exports.up = function (db, cb) {
+    db.runSql(
+        `
+        CREATE TABLE IF NOT EXISTS ai_chats
+        (
+            id BIGSERIAL PRIMARY KEY NOT NULL,
+            user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+            created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT now(),
+            chat JSONB NOT NULL
+        );
+
+        CREATE INDEX IF NOT EXISTS idx_ai_chats_user_id ON ai_chats(user_id);
+        `,
+        cb,
+    );
+};
+
+exports.down = function (db, cb) {
+    db.runSql(
+        `
+        DROP INDEX IF EXISTS idx_ai_chats_user_id;
+        DROP TABLE IF EXISTS ai_chats;
+        `,
+        cb,
+    );
+};