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/model/cdn.go

@@ -0,0 +1,8 @@
+package model
+
+type CDNInstance struct {
+	ID     string `json:"id"     table:"ID"`
+	Domain string `json:"domain" table:"DOMAIN"`
+	Status string `json:"status" table:"STATUS"`
+	Raw    any    `json:"-"`
+}