Rename ISPmail models to use Virtual* prefix

- Domain -> VirtualDomain
- User -> VirtualUser
- Alias -> VirtualAlias

This clarifies which tables are ISPmail tables vs IMC tables.

backend/internal/api/handlers/stats.go

@@ -28,7 +28,7 @@ func (h *StatsHandler) Get(c *gin.Context) {
 
 	users, err := h.db.GetAllMailUsers()
 	if err != nil {
-		users = []db.User{}
+		users = []db.VirtualUser{}
 	}
 
 	aliases, err := h.db.GetAllAliases()