perf(estimator): executemany batch insert in _save_yandex_history_items (Refs #826)

Replace N+1 loop `for row in rows: db.execute(sql, row)` with single
`db.execute(sql, rows)` — SQLAlchemy Session passes list-of-dicts as
executemany, reducing round-trips to one per batch. Guard with `if rows:`
so empty batches skip the execute. Update unit tests to reflect one call
with a list instead of N calls with individual dicts.

tradein-mvp/backend/app/services/estimator.py

@@ -659,8 +659,8 @@ def _save_yandex_history_items(
     )
 
     try:
-        for row in rows:
-            db.execute(sql, row)
+        if rows:
+            db.execute(sql, rows)
         db.commit()
         return len(rows)
     except Exception as e:

tradein-mvp/backend/tests/test_yandex_valuation_save.py

@@ -84,10 +84,11 @@ def test_save_row_contains_house_id_and_confidence():
     ):
         _save_yandex_history_items(db, result)
 
-    # db.execute called twice (one per item)
-    assert db.execute.call_count == 2
-    for c in db.execute.call_args_list:
-        row = c.args[1]
+    # db.execute called once with list-of-dicts (executemany, one round-trip)
+    assert db.execute.call_count == 1
+    rows = db.execute.call_args_list[0].args[1]
+    assert isinstance(rows, list) and len(rows) == 2
+    for row in rows:
         assert row["house_id"] == 54321
         assert row["confidence"] == pytest.approx(0.9)
         assert row["notes"] == "match_method=fingerprint"
@@ -117,8 +118,9 @@ def test_save_row_contains_removed_date():
     ):
         _save_yandex_history_items(db, result)
 
-    row = db.execute.call_args_list[0].args[1]
-    assert row["removed_date"] == date(2024, 5, 20)
+    # args[1] is now the list-of-dicts passed to executemany
+    rows = db.execute.call_args_list[0].args[1]
+    assert rows[0]["removed_date"] == date(2024, 5, 20)
 
 
 def test_save_row_removed_date_none_when_active():
@@ -144,8 +146,8 @@ def test_save_row_removed_date_none_when_active():
     ):
         _save_yandex_history_items(db, result)
 
-    row = db.execute.call_args_list[0].args[1]
-    assert row["removed_date"] is None
+    rows = db.execute.call_args_list[0].args[1]
+    assert rows[0]["removed_date"] is None
 
 
 def test_save_handles_match_failure_gracefully():
@@ -172,10 +174,10 @@ def test_save_handles_match_failure_gracefully():
         saved = _save_yandex_history_items(db, result)
 
     assert saved == 1
-    row = db.execute.call_args_list[0].args[1]
-    assert row["house_id"] is None
-    assert row["confidence"] == pytest.approx(0.0)
-    assert row["notes"] is None
+    rows = db.execute.call_args_list[0].args[1]
+    assert rows[0]["house_id"] is None
+    assert rows[0]["confidence"] == pytest.approx(0.0)
+    assert rows[0]["notes"] is None
 
 
 def test_save_match_called_with_address_and_year():