Add opportunity detection and storage functionality with async processing

- Introduced OpportunityEvent class for structured opportunity data.
- Enhanced IncrementalCycleDetector to generate opportunities based on updated pairs.
- Implemented AsyncOpportunityWriter for persisting opportunities to the database.
- Updated MarketDataFeed to handle opportunity detection and execution in both paper and live trading modes.
- Added unit tests for opportunity detection and persistence.

tests/unit/test_opportunity_writer.py

@@ -0,0 +1,48 @@
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+import pytest
+
+from arbitrade.config.settings import Settings
+from arbitrade.detection.engine import OpportunityEvent
+from arbitrade.storage.db import DuckDBStore
+from arbitrade.storage.opportunities import AsyncOpportunityWriter
+from arbitrade.storage.repositories import OpportunityRepository
+
+
+@pytest.mark.asyncio
+async def test_async_opportunity_writer_persists_events(tmp_path) -> None:
+    settings = Settings(_env_file=None, DUCKDB_PATH=tmp_path / "test.duckdb")
+    store = DuckDBStore(settings)
+    store.migrate()
+
+    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()
+
+    with store.connect() as conn:
+        rows = conn.execute(
+            "SELECT cycle, gross_pct, net_pct, est_profit, executed FROM opportunities"
+        ).fetchall()
+
+    assert len(rows) == 1
+    assert rows[0][0] == "USD->BTC->ETH->USD"
+    assert rows[0][1] == 4.0
+    assert rows[0][2] == 3.0
+    assert rows[0][3] == 0.03
+    assert rows[0][4] is False