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/server.go

@@ -5,7 +5,7 @@ import (
 	"fmt"
 	"time"
 
-	"git.yoresee.cc/nightstar/ry/internal/provider"
+	"git.yoresee.cc/nightstar/ry/internal/model"
 )
 
 // ServerService implements provider.ServerService for 雨云.
@@ -27,19 +27,19 @@ type rainyunServer struct {
 	ExpireDate int64  `json:"ExpDate"` // Unix timestamp
 }
 
-func (s *ServerService) List(ctx context.Context) ([]provider.Server, error) {
+func (s *ServerService) List(ctx context.Context) ([]model.Server, error) {
 	var raw []rainyunServer
 	if err := s.client.getList(ctx, "/product/rcs/", &raw); err != nil {
 		return nil, err
 	}
-	servers := make([]provider.Server, len(raw))
+	servers := make([]model.Server, len(raw))
 	for i, r := range raw {
 		servers[i] = toServer(r)
 	}
 	return servers, nil
 }
 
-func (s *ServerService) Get(ctx context.Context, id string) (*provider.Server, error) {
+func (s *ServerService) Get(ctx context.Context, id string) (*model.Server, error) {
 	var raw rainyunServer
 	path := fmt.Sprintf("/product/rcs/%s/", id)
 	if err := s.client.get(ctx, path, &raw); err != nil {
@@ -85,10 +85,10 @@ func (s *ServerService) Upgrade(ctx context.Context, id, plan string) error {
 	return s.client.post(ctx, fmt.Sprintf("/product/rcs/%s/upgrade", id), body, nil)
 }
 
-// toServer maps a rainyunServer to the provider.Server model.
-func toServer(r rainyunServer) provider.Server {
+// toServer maps a rainyunServer to the model.Server model.
+func toServer(r rainyunServer) model.Server {
 	expire := time.Unix(r.ExpireDate, 0)
-	return provider.Server{
+	return model.Server{
 		ID:       fmt.Sprintf("%d", r.ID),
 		Name:     r.Name,
 		Status:   r.Status,