feat: enforce session_id persistence in trade ledger (TASK-CODE-007)

tests/test_db.py

@@ -155,6 +155,7 @@ def test_mode_column_exists_in_schema() -> None:
     cursor = conn.execute("PRAGMA table_info(trades)")
     columns = {row[1] for row in cursor.fetchall()}
     assert "mode" in columns
+    assert "session_id" in columns
     assert "strategy_pnl" in columns
     assert "fx_pnl" in columns
 
@@ -199,15 +200,17 @@ def test_mode_migration_adds_column_to_existing_db() -> None:
         cursor = conn.execute("PRAGMA table_info(trades)")
         columns = {row[1] for row in cursor.fetchall()}
         assert "mode" in columns
+        assert "session_id" in columns
         assert "strategy_pnl" in columns
         assert "fx_pnl" in columns
         migrated = conn.execute(
-            "SELECT pnl, strategy_pnl, fx_pnl FROM trades WHERE stock_code='AAPL' LIMIT 1"
+            "SELECT pnl, strategy_pnl, fx_pnl, session_id FROM trades WHERE stock_code='AAPL' LIMIT 1"
         ).fetchone()
         assert migrated is not None
         assert migrated[0] == 123.45
         assert migrated[1] == 123.45
         assert migrated[2] == 0.0
+        assert migrated[3] == "UNKNOWN"
         conn.close()
     finally:
         os.unlink(db_path)
@@ -277,3 +280,20 @@ def test_log_trade_partial_fx_input_does_not_infer_negative_strategy_pnl() -> No
     assert row[0] == 10.0
     assert row[1] == 0.0
     assert row[2] == 10.0
+
+
+def test_log_trade_persists_explicit_session_id() -> None:
+    conn = init_db(":memory:")
+    log_trade(
+        conn=conn,
+        stock_code="AAPL",
+        action="BUY",
+        confidence=70,
+        rationale="session test",
+        market="US_NASDAQ",
+        exchange_code="NASD",
+        session_id="US_PRE",
+    )
+    row = conn.execute("SELECT session_id FROM trades ORDER BY id DESC LIMIT 1").fetchone()
+    assert row is not None
+    assert row[0] == "US_PRE"