ver. 0.304: Configurable payment method order amount limits

Replace hardcoded PayPo condition (id=6, 40-1000 PLN) with generic
min/max order amount columns on pp_shop_payment_methods. Admin form
fields added, frontend basket checkout filters dynamically. Cache
invalidation on save. 4 new tests (734 total, 2080 assertions).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

migrations/0.304.sql

@@ -0,0 +1,2 @@
+ALTER TABLE pp_shop_payment_methods ADD COLUMN min_order_amount DECIMAL(10,2) DEFAULT NULL;
+ALTER TABLE pp_shop_payment_methods ADD COLUMN max_order_amount DECIMAL(10,2) DEFAULT NULL;