feat: add Remnashop migration import

Add the Remnashop legacy importer, compatibility tables, admin toggles, referral and promo lookup compatibility, and tests for the migration flow.

tests/test_remnashop_import.py

@@ -0,0 +1,44 @@
+from datetime import datetime, timezone
+
+from scripts.import_legacy import (
+    remnashop_months_from_plan_snapshot,
+    remnashop_pricing_amount,
+    remnashop_pricing_currency,
+    remnashop_sale_mode,
+    remnashop_traffic_gb_to_bytes,
+    remnashop_transaction_status,
+)
+
+
+def test_remnashop_pricing_helpers_read_final_amount_and_currency():
+    pricing = {"final_amount": "199.50", "currency": "rub"}
+
+    assert remnashop_pricing_amount(pricing) == 199.5
+    assert remnashop_pricing_currency(pricing) == "RUB"
+
+
+def test_remnashop_traffic_limit_is_converted_from_gib():
+    assert remnashop_traffic_gb_to_bytes(10) == 10 * 1024**3
+    assert remnashop_traffic_gb_to_bytes(None) is None
+
+
+def test_remnashop_status_and_sale_mode_mapping_matches_current_payment_model():
+    assert remnashop_transaction_status("COMPLETED", "YOOKASSA") == "succeeded"
+    assert remnashop_transaction_status("PENDING", "WATA") == "pending_wata"
+    assert remnashop_transaction_status("CANCELED", "WATA") == "canceled"
+    assert remnashop_sale_mode("NEW") == "subscription"
+    assert remnashop_sale_mode("RENEW") == "subscription"
+    assert remnashop_sale_mode("CHANGE") == "tariff_upgrade"
+
+
+def test_remnashop_plan_months_prefers_snapshot_then_dates():
+    assert remnashop_months_from_plan_snapshot({"duration_days": 90}) == 3
+    assert remnashop_months_from_plan_snapshot({"months": 12}) == 12
+    assert (
+        remnashop_months_from_plan_snapshot(
+            {},
+            created_at=datetime(2026, 1, 1, tzinfo=timezone.utc),
+            expire_at=datetime(2026, 4, 1, tzinfo=timezone.utc),
+        )
+        == 3
+    )