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/domain.go

@@ -4,7 +4,7 @@ import (
 	"context"
 	"fmt"
 
-	"git.yoresee.cc/nightstar/ry/internal/provider"
+	"git.yoresee.cc/nightstar/ry/internal/model"
 )
 
 type DomainService struct {
@@ -18,14 +18,14 @@ type rainyunDomain struct {
 	ExpireDate string `json:"expire_date"`
 }
 
-func (d *DomainService) List(ctx context.Context) ([]provider.Domain, error) {
+func (d *DomainService) List(ctx context.Context) ([]model.Domain, error) {
 	var raw []rainyunDomain
 	if err := d.client.getList(ctx, "/product/domain/", &raw); err != nil {
 		return nil, err
 	}
-	domains := make([]provider.Domain, len(raw))
+	domains := make([]model.Domain, len(raw))
 	for i, r := range raw {
-		domains[i] = provider.Domain{
+		domains[i] = model.Domain{
 			ID:   fmt.Sprintf("%d", r.ID),
 			Name: r.DomainName,
 		}
@@ -33,7 +33,7 @@ func (d *DomainService) List(ctx context.Context) ([]provider.Domain, error) {
 	return domains, nil
 }
 
-func (d *DomainService) DNSList(ctx context.Context, domain string) ([]provider.DNSRecord, error) {
+func (d *DomainService) DNSList(ctx context.Context, domain string) ([]model.DNSRecord, error) {
 	type rainyunDNSRecord struct {
 		ID         int    `json:"id"`
 		RecordType string `json:"record_type"`
@@ -46,9 +46,9 @@ func (d *DomainService) DNSList(ctx context.Context, domain string) ([]provider.
 	if err := d.client.get(ctx, path, &raw); err != nil {
 		return nil, err
 	}
-	records := make([]provider.DNSRecord, len(raw))
+	records := make([]model.DNSRecord, len(raw))
 	for i, r := range raw {
-		records[i] = provider.DNSRecord{
+		records[i] = model.DNSRecord{
 			ID:    fmt.Sprintf("%d", r.ID),
 			Type:  r.RecordType,
 			Name:  r.Host,
@@ -59,7 +59,7 @@ func (d *DomainService) DNSList(ctx context.Context, domain string) ([]provider.
 	return records, nil
 }
 
-func (d *DomainService) DNSAdd(ctx context.Context, domain string, record provider.DNSRecord) error {
+func (d *DomainService) DNSAdd(ctx context.Context, domain string, record model.DNSRecord) error {
 	body := map[string]any{
 		"record_type": record.Type,
 		"host":        record.Name,