fix: parameterize import snapshot date filters

tests/test_import_service_sql_params.py

@@ -1,5 +1,7 @@
 from pathlib import Path
+from datetime import date
 
+from services import import_service
 from services.import_service import _build_in_clause
 
 
@@ -17,6 +19,20 @@ def test_import_service_does_not_interpolate_date_values_into_in_clauses():
     assert "join([f\"'{d}'\" for d in" not in source
 
 
+def test_daily_snapshot_delete_casts_text_date_column_on_postgres(monkeypatch):
+    monkeypatch.setattr(import_service, "_db_dialect_name", lambda: "postgresql")
+
+    assert import_service._date_filter_expr("snapshot_date") == "snapshot_date::date"
+    assert import_service._normalise_date_values_for_sql(["2026-05-01"]) == [date(2026, 5, 1)]
+
+
+def test_daily_snapshot_delete_uses_iso_dates_on_sqlite(monkeypatch):
+    monkeypatch.setattr(import_service, "_db_dialect_name", lambda: "sqlite")
+
+    assert import_service._date_filter_expr("snapshot_date") == "date(snapshot_date)"
+    assert import_service._normalise_date_values_for_sql([date(2026, 5, 1)]) == ["2026-05-01"]
+
+
 def test_monthly_summary_import_does_not_replace_entire_table():
     source = Path("routes/import_routes.py").read_text(encoding="utf-8")
     start = source.index("DELETE FROM monthly_summary_analysis WHERE year = :y AND month = :m")