feat: NATS JetStream + Todoist background sync (#21, #22)

Issue 21 — event infrastructure:
- NormalizedEvent<T> + payload types in packages/shared-types/src/events/
- Bus.onPublish() hook for side-effect bridges
- NATS JetStream adapter (services/api/src/events/nats.ts): connects when
  NATS_URL is set, creates signals.> and feedback.> streams, bridges all
  in-process bus publishes to JetStream — no-ops gracefully when NATS is absent
- NATS service added to docker-compose (profile: events|full, port 4222/8222)

Issue 22 — Todoist background sync:
- services/api/src/signals/scheduler.ts: queries all active-token users every
  15 min (TODOIST_SYNC_INTERVAL_MS), fan-out via todoistSource.fetchSignals()
  which emits signals.task.synced; on-demand fetch remains as freshness fallback
- NATS_URL + TODOIST_SYNC_INTERVAL_MS added to config

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/api/src/index.ts

@@ -18,6 +18,8 @@ import { dirname } from 'path';
 import { requireAuth } from './middleware/session.js';
 import { requireAdmin } from './middleware/admin.js';
 import type { Request, Response } from 'express';
+import { connectNats } from './events/nats.js';
+import { startTodoistSyncScheduler } from './signals/scheduler.js';
 
 await mkdir(dirname(config.DATABASE_PATH), { recursive: true });
 runMigrations();
@@ -88,3 +90,9 @@ setInterval(purgeExpiredData, 24 * 60 * 60 * 1000);
 app.listen(config.PORT, () => {
   console.log(`oO API listening on http://localhost:${config.PORT}`);
 });
+
+if (config.NATS_URL) {
+  connectNats(config.NATS_URL);
+}
+
+startTodoistSyncScheduler(config.TODOIST_SYNC_INTERVAL_MS);