refactor: extract domain models from provider into internal/model

- Move Server/Domain/DNSRecord/Bucket/StorageInstance/CDNInstance/Order/Error structs to internal/model/
- Provider interfaces now reference model.* types
- Rainyun provider implementations import model package for data types
- CLI commands updated to use model.DNSRecord directly

Separates pure data structures from service contract interfaces,
following hexagonal architecture: models (entities) live independently
from ports (provider interfaces) and adapters (rainyun/* implementations).

internal/provider/rainyun/billing.go

@@ -5,14 +5,14 @@ import (
 	"fmt"
 	"time"
 
-	"git.yoresee.cc/nightstar/ry/internal/provider"
+	"git.yoresee.cc/nightstar/ry/internal/model"
 )
 
 type BillingService struct {
 	client *client
 }
 
-func (b *BillingService) Orders(ctx context.Context) ([]provider.Order, error) {
+func (b *BillingService) Orders(ctx context.Context) ([]model.Order, error) {
 	type rainyunOrder struct {
 		ID          int     `json:"id"`
 		ProductName string  `json:"product_name"`
@@ -24,10 +24,10 @@ func (b *BillingService) Orders(ctx context.Context) ([]provider.Order, error) {
 	if err := b.client.getList(ctx, "/expense/orders/list", &raw); err != nil {
 		return nil, err
 	}
-	orders := make([]provider.Order, len(raw))
+	orders := make([]model.Order, len(raw))
 	for i, r := range raw {
 		t, _ := time.Parse("2006-01-02 15:04:05", r.CreateDate)
-		orders[i] = provider.Order{
+		orders[i] = model.Order{
 			ID:      fmt.Sprintf("%d", r.ID),
 			Product: r.ProductName,
 			Amount:  r.Amount,