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/internal/service/account_stats_pricing_test.go

@@ -147,14 +147,14 @@ func TestFindPricingForModel(t *testing.T) {
 			wantNil: true,
 		},
 		{
-			name: "wildcard matches by longest prefix (most specific wins)",
+			name: "wildcard matches by config order (first match wins)",
 			list: []ChannelModelPricing{
 				{ID: 10, Models: []string{"claude-*"}},
 				{ID: 11, Models: []string{"claude-opus-*"}},
 			},
 			platform: "",
 			model:    "claude-opus-4",
-			wantID:   11, // "claude-opus-*" is longer prefix, wins over "claude-*"
+			wantID:   10, // config order: "claude-*" is first and matches, so it wins
 		},
 		{
 			name: "shorter wildcard used when longer does not match",