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/paymentproviderinstance/paymentproviderinstance.go

@@ -31,6 +31,8 @@ const (
 	FieldLimits = "limits"
 	// FieldRefundEnabled holds the string denoting the refund_enabled field in the database.
 	FieldRefundEnabled = "refund_enabled"
+	// FieldAllowUserRefund holds the string denoting the allow_user_refund field in the database.
+	FieldAllowUserRefund = "allow_user_refund"
 	// FieldCreatedAt holds the string denoting the created_at field in the database.
 	FieldCreatedAt = "created_at"
 	// FieldUpdatedAt holds the string denoting the updated_at field in the database.
@@ -51,6 +53,7 @@ var Columns = []string{
 	FieldSortOrder,
 	FieldLimits,
 	FieldRefundEnabled,
+	FieldAllowUserRefund,
 	FieldCreatedAt,
 	FieldUpdatedAt,
 }
@@ -88,6 +91,8 @@ var (
 	DefaultLimits string
 	// DefaultRefundEnabled holds the default value on creation for the "refund_enabled" field.
 	DefaultRefundEnabled bool
+	// DefaultAllowUserRefund holds the default value on creation for the "allow_user_refund" field.
+	DefaultAllowUserRefund bool
 	// DefaultCreatedAt holds the default value on creation for the "created_at" field.
 	DefaultCreatedAt func() time.Time
 	// DefaultUpdatedAt holds the default value on creation for the "updated_at" field.
@@ -149,6 +154,11 @@ func ByRefundEnabled(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldRefundEnabled, opts...).ToFunc()
 }
 
+// ByAllowUserRefund orders the results by the allow_user_refund field.
+func ByAllowUserRefund(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldAllowUserRefund, opts...).ToFunc()
+}
+
 // ByCreatedAt orders the results by the created_at field.
 func ByCreatedAt(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldCreatedAt, opts...).ToFunc()