fix(signals): add missing source field to TaskSyncedEvent (#78)

TaskSyncedPayload in shared-types and ml/serving schemas both require
source, but TaskSyncedEvent in bus.ts and the todoist publish call both
omitted it — causing the JetStream consumer to nak every task.synced
message on validation failure.

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

services/api/src/signals/todoist.ts

@@ -88,7 +88,7 @@ export class TodoistSignalSource implements SignalSource {
     });
 
     this.cache.set(userId, { signals, fetchedAt: Date.now() });
-    bus.publish('signals.task.synced', { userId, count: signals.length, syncedAt: now });
+    bus.publish('signals.task.synced', { userId, source: 'todoist', count: signals.length, syncedAt: now });
 
     return signals;
   }