feat: SignalSource abstraction — generalize signal ingestion beyond Todoist (#78)

- Add Signal + SignalSource interfaces to packages/shared-types
- TipCandidate.features widened to Record<string,number|boolean> to match Signal
- TodoistSignalSource: encapsulates fetch, cache, 401 handling, bus events, and act()
- SignalAggregator: parallel fan-out across sources with per-source failure isolation
- Recommender refactored to consume Signal[] via aggregator; source action dispatch via aggregator.act()
- ADR-0009: signal normalization strategy

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

services/api/src/routes/__tests__/recommender.test.ts

@@ -62,7 +62,7 @@ describe('POST /recommend integration', () => {
 
     const mod = await import('../recommender.js');
     const { recommenderRouter } = mod;
-    clearCache = (mod as any)._clearTaskCacheForTests;
+    clearCache = (mod as any)._clearCandidateCacheForTests;
     const app = express();
     app.use(express.json());
     app.use('/api', recommenderRouter);