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- Removed outdated unit tests that relied on DuckDB and replaced them with tests using PgStore.

.gitignore

@@ -38,6 +38,7 @@ dist/
 # Local database / runtime data
 data/*.duckdb.wal
 data/*.duckdb.tmp
+data/arbitrade.duckdb
 logs/
 ops/performance/latest_profile.json