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>

infra/docker/docker-compose.yml

@@ -178,6 +178,26 @@ services:
       timeout: 5s
       retries: 5
 
+  # ── events profile — NATS JetStream ─────────────────────────────────────
+  # Start: docker compose --profile events up
+  # NATS monitoring: http://localhost:8222
+  # Enable in the API by setting NATS_URL=nats://nats:4222 in .env.local
+
+  nats:
+    image: nats:2.10-alpine
+    profiles: [events, full]
+    command: ["-js", "-sd", "/data", "-m", "8222"]
+    volumes:
+      - /mnt/ssd/dbs/oo/nats:/data
+    ports:
+      - "127.0.0.1:4222:4222"   # client connections
+      - "127.0.0.1:8222:8222"   # HTTP monitoring
+    healthcheck:
+      test: ["CMD", "wget", "--spider", "-q", "http://localhost:8222/healthz"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+
   litellm:
     image: ghcr.io/berriai/litellm:main-latest
     profiles: [ai]