feat(kevin): SA models for bridge audit + backtest persistence

3 tables (kevin_signal_bridge_state, kevin_backtest_runs,
kevin_backtest_trades) all UUID-keyed for consistency with Trade/Position.
KEVIN_STRATEGY_UUID constant pinned for FK joins from Trade.strategy_id.

tests/conftest.py

@@ -0,0 +1,49 @@
+"""Shared pytest fixtures for the trading-bot test suite.
+
+Provides:
+- `db_session`: an `AsyncSession` against the dev Postgres (transaction-scoped,
+  rolled back between tests).
+"""
+
+from __future__ import annotations
+
+import os
+from collections.abc import AsyncIterator
+
+import pytest_asyncio
+from sqlalchemy.ext.asyncio import (
+    AsyncSession,
+    async_sessionmaker,
+    create_async_engine,
+)
+
+
+def _dev_db_url() -> str:
+    return os.environ.get(
+        "TRADING_TEST_DATABASE_URL",
+        "postgresql+asyncpg://trading:trading@localhost:5432/trading_dev",
+    )
+
+
+@pytest_asyncio.fixture
+async def db_session() -> AsyncIterator[AsyncSession]:
+    """Per-test session wrapped in a transaction that is rolled back at teardown."""
+    engine = create_async_engine(_dev_db_url(), echo=False)
+    try:
+        connection = await engine.connect()
+        transaction = await connection.begin()
+        session_factory = async_sessionmaker(
+            bind=connection,
+            class_=AsyncSession,
+            expire_on_commit=False,
+            join_transaction_mode="create_savepoint",
+        )
+        session = session_factory()
+        try:
+            yield session
+        finally:
+            await session.close()
+            await transaction.rollback()
+            await connection.close()
+    finally:
+        await engine.dispose()