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/domain.go

@@ -0,0 +1,19 @@
+package model
+
+import "time"
+
+type Domain struct {
+	ID       string    `json:"id"       table:"ID"`
+	Name     string    `json:"name"     table:"DOMAIN"`
+	Status   string    `json:"status"   table:"STATUS"`
+	ExpireAt time.Time `json:"expires"  table:"EXPIRES"`
+	Raw      any       `json:"-"`
+}
+
+type DNSRecord struct {
+	ID    string `json:"id"    table:"ID"`
+	Type  string `json:"type"  table:"TYPE"`
+	Name  string `json:"name"  table:"NAME"`
+	Value string `json:"value" table:"VALUE"`
+	TTL   int    `json:"ttl"   table:"TTL"`
+}