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/billing.go

@@ -0,0 +1,12 @@
+package model
+
+import "time"
+
+type Order struct {
+	ID      string    `json:"id"      table:"ID"`
+	Product string    `json:"product" table:"PRODUCT"`
+	Amount  float64   `json:"amount"  table:"AMOUNT"`
+	Status  string    `json:"status"  table:"STATUS"`
+	Created time.Time `json:"created" table:"CREATED"`
+	Raw     any       `json:"-"`
+}