test: verify Next.js application works with PostgreSQL

- Updated .env to use DATABASE_URL instead of DATABASE_PATH
- Tested all API endpoints: health, charts, candles, span annotations
- Confirmed JSONB fields work correctly (geometry, sub_spans, model_prediction)
- All 2,836 rows accessible via API
- Database connection pooling working correctly

.env

@@ -1,6 +1,6 @@
 NODE_ENV=production
 PORT=3000
-DATABASE_PATH=/app/data/candles.db
+DATABASE_URL=postgresql://ml_user:ml_password@localhost:5432/candle_annotator
 
 # ML Inference Service Configuration
 INFERENCE_API_URL=http://localhost:8001