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/server.go

@@ -0,0 +1,17 @@
+package model
+
+import "time"
+
+type Server struct {
+	ID       string    `json:"id"       table:"ID"`
+	Name     string    `json:"name"     table:"NAME"`
+	Status   string    `json:"status"   table:"STATUS"`
+	IP       string    `json:"ip"       table:"IP"`
+	CPU      int       `json:"cpu"      table:"CPU"`
+	Memory   int       `json:"memory"   table:"MEM"`
+	Disk     int       `json:"disk"     table:"DISK"`
+	OS       string    `json:"os"       table:"OS"`
+	Region   string    `json:"region"   table:"REGION"`
+	ExpireAt time.Time `json:"expires"  table:"EXPIRES"`
+	Raw      any       `json:"-"`
+}