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- Removed outdated unit tests that relied on DuckDB and replaced them with tests using PgStore.

tests/integration/conftest.py

@@ -0,0 +1,25 @@
+"""pytest configuration for integration tests.
+
+Integration tests require a live PostgreSQL server at the configured host.
+They are skipped automatically if the server is unreachable.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+
+def pytest_ignore_collect(path: str, config: pytest.Config) -> bool:
+    """Skip integration tests unless --integration is passed."""
+    if "integration" in path and not config.getoption("--integration", False):
+        return True
+    return False
+
+
+def pytest_addoption(parser: pytest.Parser) -> None:
+    parser.addoption(
+        "--integration",
+        action="store_true",
+        default=False,
+        help="Run integration tests (requires PostgreSQL)",
+    )