Fase 1a: driver mongodb v3.7 (produccion es MongoDB 3.2)

El smoke-test contra prod revelo MongoDB 3.2.11 (EOL): el driver v6 exige >=4.2
y no conecta. Cambios:

- mongodb ^3.7.4 + @types/mongodb; db.ts con useUnifiedTopology, sin retryWrites
- sends.ts: deteccion de duplicate-key (11000) sin depender de MongoServerError
- poller.ts: FilterQuery en vez de Filter
- tests de integracion migrados de mongodb-memory-server a un fake en memoria
  (test/fake-repo.ts) porque no hay mongod 3.x ejecutable en hosts modernos;
  emula operadores ($ne/$in/$gte/$exists), $set/$unset e indice unico (11000)
- docs: legacy-behavior.md §0 (entorno MongoDB 3.2, sin change streams -> fase
  1b usara polling; recomendacion de upgrade de Mongo como fase propia)

47 tests en verde, typecheck y build OK. Datos reales del import: 413 pendientes
FCM, 0 email, 8176 users con fireBaseToken.

test/handlers.test.ts

@@ -1,30 +1,14 @@
-import { afterAll, beforeAll, beforeEach, describe, expect, it } from 'vitest';
-import type { MongoMemoryServer } from 'mongodb-memory-server';
+import { beforeEach, describe, expect, it } from 'vitest';
 import type { Repos } from '../src/db.js';
 import type { Config } from '../src/config.js';
 import { processEmailJob, processFcmJob, type Deps } from '../src/handlers.js';
-import { aNotif, aUser, fakeFcm, fakeMailer, silentLog, startMongo } from './helpers.js';
+import { aNotif, aUser, fakeFcm, fakeMailer, freshRepos, silentLog, testCfg } from './helpers.js';
 
-let server: MongoMemoryServer;
 let repos: Repos;
-let baseCfg: Config;
-
-beforeAll(async () => {
-  const s = await startMongo();
-  server = s.server;
-  repos = s.repos;
-  baseCfg = s.cfg;
-});
-
-afterAll(async () => {
-  await repos.close();
-  await server.stop();
-});
+const baseCfg: Config = testCfg();
 
 beforeEach(async () => {
-  await repos.notifications.deleteMany({});
-  await repos.users.deleteMany({});
-  await repos.sends.deleteMany({});
+  repos = await freshRepos();
 });
 
 function deps(over: Partial<Deps> = {}): Deps {