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- Removed outdated unit tests that relied on DuckDB and replaced them with tests using PgStore.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

@@ -0,0 +1,61 @@
+from __future__ import annotations
+
+from collections.abc import AsyncIterator
+from contextlib import asynccontextmanager
+from datetime import UTC, datetime
+
+import pytest
+
+from arbitrade.config.settings import get_settings
+from arbitrade.detection.engine import OpportunityEvent
+from arbitrade.storage.pg_store import PgStore
+from arbitrade.storage.opportunities import AsyncOpportunityWriter
+from arbitrade.storage.repositories import OpportunityRepository
+
+pytestmark = pytest.mark.integration
+
+
+@asynccontextmanager
+async def _pg() -> AsyncIterator[PgStore]:
+    s = get_settings()
+    store = PgStore(s)
+    try:
+        await store.start()
+        await store.migrate()
+        yield store
+    finally:
+        await store.stop()
+
+
+@pytest.mark.asyncio
+async def test_async_opportunity_writer_persists_events() -> None:
+    async with _pg() as store:
+        repository = OpportunityRepository(store)
+        writer = AsyncOpportunityWriter(repository, max_queue_size=10)
+        await writer.start()
+
+        event = OpportunityEvent(
+            detected_at=datetime.now(UTC),
+            cycle="USD->BTC->ETH->USD",
+            updated_pair="BTC/USD",
+            gross_rate=1.04,
+            net_rate=1.03,
+            gross_pct=4.0,
+            net_pct=3.0,
+            est_profit=0.03,
+        )
+
+        await writer.enqueue(event)
+        await writer.stop()
+
+        async with store.pool.acquire() as conn:
+            rows = await conn.fetch(
+                "SELECT cycle, gross_pct, net_pct, est_profit, executed FROM opportunities"
+            )
+
+        assert len(rows) == 1
+        assert rows[0]["cycle"] == "USD->BTC->ETH->USD"
+        assert rows[0]["gross_pct"] == 4.0
+        assert rows[0]["net_pct"] == 3.0
+        assert rows[0]["est_profit"] == 0.03
+        assert rows[0]["executed"] is False