Add backend validation for aliases

- Validate source email format (RFC 5321 local-part)
- Ensure source is in the correct domain
- Validate destination (single or comma-separated emails)
- Check for duplicate aliases
- Add GetAliasBySource database method

backend/internal/db/aliases.go

@@ -42,6 +42,14 @@ func (d *DB) GetAliasByID(id uint) (*Alias, error) {
 	return &alias, nil
 }
 
+func (d *DB) GetAliasBySource(source string) (*Alias, error) {
+	var alias Alias
+	if err := d.Where("source = ?", source).First(&alias).Error; err != nil {
+		return nil, err
+	}
+	return &alias, nil
+}
+
 func (d *DB) CreateAlias(source, destination string) (*Alias, error) {
 	alias := Alias{
 		Source:      source,