Add integration tests for execution persistence, metrics, and opportunity writing

- Implemented integration tests for the execution writer to ensure trade orders and PnL are persisted correctly.
- Created integration tests for the metrics calculator to summarize execution data accurately.
- Added integration tests for the opportunity writer to verify event persistence.
- Established PostgreSQL schema validation tests to ensure all expected tables, columns, and constraints exist.
- Removed outdated unit tests that relied on DuckDB and replaced them with tests using PgStore.

ops/performance/README.md

@@ -39,7 +39,7 @@ Key end-to-end latency baselines from `latency_baseline.json`:
 
 ## Optimization Note
 
-`MetricsCalculator.compute()` was optimized to use DuckDB SQL aggregations and quantiles, reducing Python-side row scans.
+`MetricsCalculator.compute()` uses PostgreSQL SQL aggregations and percentiles, reducing Python-side row scans.
 
 Measured benchmark (`scripts/benchmark_metrics_compute.py`):