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>

packages/shared-types/src/events/index.ts

@@ -0,0 +1,60 @@
+/**
+ * NormalizedEvent — the durable envelope for all events flowing through
+ * the system. Today: in-process EventEmitter. Tomorrow: NATS JetStream.
+ *
+ * Subject taxonomy:
+ *   signals.task.synced      — Todoist (or other source) task list refreshed
+ *   signals.tip.served       — tip returned to client
+ *   signals.tip.feedback     — user reacted (done / dismiss / snooze / helpful / not_helpful)
+ *   signals.tip.reward_failed — reward delivery to ml/serving failed after retries
+ *   signals.integration.token_expired — OAuth token needs reconnect
+ */
+export interface NormalizedEvent<T = unknown> {
+  /** NATS-style subject: domain.entity.verb */
+  subject: string;
+  /** ISO 8601 timestamp */
+  ts: string;
+  /** Monotonically increasing sequence number (in-process ring; JetStream seq in prod) */
+  seq: number;
+  payload: T;
+}
+
+// ── Payload types ────────────────────────────────────────────────────────────
+
+export interface TaskSyncedPayload {
+  userId: string;
+  source: string;   // e.g. 'todoist'
+  count: number;
+  syncedAt: string;
+}
+
+export interface TipServedPayload {
+  userId: string;
+  tipId: string;
+  policy: string;
+  servedAt: string;
+}
+
+export interface TipFeedbackPayload {
+  userId: string;
+  tipId: string;
+  action: 'done' | 'dismiss' | 'snooze' | 'helpful' | 'not_helpful';
+  reward: number;
+  dwellMs: number | null;
+  createdAt: string;
+}
+
+export interface TipRewardFailedPayload {
+  userId: string;
+  tipId: string;
+  reward: number;
+  attempts: number;
+  error: string;
+  failedAt: string;
+}
+
+export interface IntegrationTokenExpiredPayload {
+  userId: string;
+  provider: string;
+  detectedAt: string;
+}

packages/shared-types/src/index.ts

@@ -3,3 +3,4 @@ export * from './http/auth.js';
 export * from './http/integrations.js';
 export * from './http/user.js';
 export * from './http/signal.js';
+export * from './events/index.js';