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/signals/scheduler.ts

@@ -0,0 +1,55 @@
+/**
+ * Todoist background sync scheduler (Issue #22).
+ *
+ * Periodically fetches Todoist tasks for every user with an active token so
+ * that signals are fresh before the next /recommend call. The on-demand fetch
+ * in TodoistSignalSource remains as the freshness-critical fallback — if a user
+ * hits /recommend and the cache is stale, it re-fetches inline.
+ *
+ * Interval: TODOIST_SYNC_INTERVAL_MS (default 15 min).
+ */
+
+import { db } from '../db/index.js';
+import { integrationTokens } from '../db/schema.js';
+import { eq } from 'drizzle-orm';
+import { todoistSource } from './todoist.js';
+
+const DEFAULT_INTERVAL_MS = 15 * 60 * 1000;
+
+export function startTodoistSyncScheduler(intervalMs = DEFAULT_INTERVAL_MS): NodeJS.Timeout {
+  async function syncAll(): Promise<void> {
+    let users: { userId: string }[] = [];
+    try {
+      users = await db
+        .select({ userId: integrationTokens.userId })
+        .from(integrationTokens)
+        .where(eq(integrationTokens.tokenStatus, 'active'));
+    } catch (err: any) {
+      console.error(`[scheduler] failed to query users: ${err.message}`);
+      return;
+    }
+
+    if (!users.length) return;
+
+    const results = await Promise.allSettled(
+      users.map((u) => todoistSource.fetchSignals(u.userId)),
+    );
+
+    let ok = 0;
+    let failed = 0;
+    for (const r of results) {
+      if (r.status === 'fulfilled') ok++;
+      else { failed++; console.error(`[scheduler] sync error:`, r.reason); }
+    }
+
+    console.log(`[scheduler] todoist sync: ${ok} ok, ${failed} failed (${users.length} users)`);
+  }
+
+  // Run once shortly after startup, then on interval
+  const delay = setTimeout(() => {
+    syncAll();
+    setInterval(syncAll, intervalMs);
+  }, 10_000);
+
+  return delay;
+}