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/domains.go

@@ -46,8 +46,8 @@ func (h *DomainHandler) List(c *gin.Context) {
 	domainStats := make([]db.DomainStats, len(domains))
 	for i, d := range domains {
 		var userCount, aliasCount int64
-		h.db.Model(&db.User{}).Where("domain_id = ?", d.ID).Count(&userCount)
-		h.db.Model(&db.Alias{}).Where("domain_id = ?", d.ID).Count(&aliasCount)
+		h.db.Model(&db.VirtualUser{}).Where("domain_id = ?", d.ID).Count(&userCount)
+		h.db.Model(&db.VirtualAlias{}).Where("domain_id = ?", d.ID).Count(&aliasCount)
 		domainStats[i] = db.DomainStats{
 			ID:         d.ID,
 			Name:       d.Name,

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()