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/events/bus.ts

@@ -69,6 +69,12 @@ const RING_SIZE = 500;
 class Bus extends EventEmitter {
   private ring: StoredEvent[] = [];
   private seq = 0;
+  private publishHooks: Array<(subject: string, payload: unknown) => void> = [];
+
+  /** Register a side-effect hook called on every publish (e.g. NATS bridge) */
+  onPublish(hook: (subject: string, payload: unknown) => void): void {
+    this.publishHooks.push(hook);
+  }
 
   publish<K extends keyof EventMap>(subject: K, payload: EventMap[K]): void {
     const entry: StoredEvent = {
@@ -80,6 +86,7 @@ class Bus extends EventEmitter {
     if (this.ring.length >= RING_SIZE) this.ring.shift();
     this.ring.push(entry);
     this.emit(subject, payload);
+    for (const hook of this.publishHooks) hook(subject, payload);
   }
 
   subscribe<K extends keyof EventMap>(subject: K, handler: (payload: EventMap[K]) => void): void {