feat: add per-provider allow_user_refund control and align wildcard matching

allow_user_refund:
- Add allow_user_refund field to PaymentProviderInstance ent schema
- Migration 103: ALTER TABLE payment_provider_instances ADD COLUMN
- Cascade logic: disabling refund_enabled auto-disables allow_user_refund
- User refund validation: check provider instance allows user refund
- Admin refund validation: check provider instance allows admin refund
- Subscription refund: deduct days on refund, rollback on failure
- New endpoint: GET /payment/orders/refund-eligible-providers
- Frontend: ToggleSwitch in ProviderCard/Dialog, cascade in SettingsView

Wildcard matching:
- Change findPricingForModel from "longest prefix wins" to "config order
  priority (first match wins)", aligning with channel service behavior

backend/ent/predicate/predicate.go

@@ -33,7 +33,6 @@ type IdempotencyRecord func(*sql.Selector)
 // PaymentAuditLog is the predicate function for paymentauditlog builders.
 type PaymentAuditLog func(*sql.Selector)
 
-
 // PaymentOrder is the predicate function for paymentorder builders.
 type PaymentOrder func(*sql.Selector)