Fase 1a: fix interop CJS/ESM del driver mongodb v3

mongodb v3.x es CommonJS; el import nombrado ESM (import { MongoClient }) no lo
resuelve el cjs-lexer de Node en runtime (SyntaxError al arrancar dist). Tomo los
valores del default export (mongodb.MongoClient / mongodb.ObjectId). Verificado
importando dist/*.js con node (no solo compilando).

src/db.ts

@@ -1,9 +1,13 @@
-import { MongoClient, type Collection, type Db } from 'mongodb';
+import mongodb, { type Collection, type Db } from 'mongodb';
 import type { Config } from './config.js';
+
+// mongodb v3.x is CommonJS; named ESM imports of its values aren't resolvable by
+// Node's cjs-lexer, so pull runtime values off the default (module.exports).
+const { MongoClient } = mongodb;
 import type { NotificationDoc, SendRecord, UserDoc } from './types.js';
 
 export interface Repos {
-  client: MongoClient;
+  client: mongodb.MongoClient;
   db: Db;
   notifications: Collection<NotificationDoc>;
   users: Collection<UserDoc>;

src/handlers.ts

@@ -1,5 +1,7 @@
-import { ObjectId } from 'mongodb';
+import mongodb from 'mongodb';
 import type { Config } from './config.js';
+// mongodb v3.x is CommonJS — take ObjectId off the default export (see db.ts).
+const { ObjectId } = mongodb;
 import type { Repos } from './db.js';
 import type { FcmClient } from './fcm.js';
 import type { Mailer } from './mailer.js';