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- Removed outdated unit tests that relied on DuckDB and replaced them with tests using PgStore.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/e2e/__init__.py

@@ -0,0 +1 @@
+"""End-to-end tests — require full app startup with PostgreSQL."""

tests/e2e/test_health.py

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+import httpx
+
+from arbitrade.api.app import create_app
+from arbitrade.config.settings import Settings
+
+
+async def test_health_route_returns_ok() -> None:
+    app = create_app(Settings())
+    transport = httpx.ASGITransport(app=app)
+    async with httpx.AsyncClient(transport=transport, base_url="http://test") as client:
+        response = await client.get("/health")
+
+    assert response.status_code == 200
+    assert response.json()["status"] == "ok"