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).

cmd/cli/commands/domain/domain.go

@@ -3,6 +3,7 @@ package domain
 import (
 	"fmt"
 
+	"git.yoresee.cc/nightstar/ry/internal/model"
 	"git.yoresee.cc/nightstar/ry/internal/output"
 	"git.yoresee.cc/nightstar/ry/internal/provider"
 
@@ -66,7 +67,7 @@ func Cmd(prov *provider.Provider, out **output.Printer) *cobra.Command {
 			value, _ := cmd.Flags().GetString("value")
 			ttl, _ := cmd.Flags().GetInt("ttl")
 			return svc.DNSAdd(cmd.Context(), args[0],
-				provider.DNSRecord{Type: recType, Name: name, Value: value, TTL: ttl})
+				model.DNSRecord{Type: recType, Name: name, Value: value, TTL: ttl})
 		},
 	}