Remove unused hostname_prefix from server types and add duplicate checking

The HostnamePrefix field on ServerType was loaded from YAML but never used —
hostnames are user-provided. This removes the field and adds explicit
duplicate checks (hostname + MAC) with clear per-field error messages in
both the JSON API and web UI, backed by a new GetByHostname store method
with case-insensitive matching.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/store/hosts.go

@@ -87,6 +87,18 @@ func (s *Hosts) GetByMAC(ctx context.Context, mac string) (*model.Host, error) {
 	return &h, nil
 }
 
+func (s *Hosts) GetByHostname(ctx context.Context, hostname string) (*model.Host, error) {
+	row := s.DB.QueryRowContext(ctx, `SELECT `+hostColumns+` FROM hosts WHERE LOWER(hostname) = ?`, normalizeHostname(hostname))
+	var h model.Host
+	if err := scanHost(row, &h); err != nil {
+		if err == sql.ErrNoRows {
+			return nil, ErrNotFound
+		}
+		return nil, fmt.Errorf("get host by hostname: %w", err)
+	}
+	return &h, nil
+}
+
 func (s *Hosts) UpdateState(ctx context.Context, id int64, state model.HostState) error {
 	res, err := s.DB.ExecContext(ctx, `UPDATE hosts SET state = ?, updated_at = strftime('%Y-%m-%dT%H:%M:%SZ','now') WHERE id = ?`, state, id)
 	if err != nil {
@@ -143,3 +155,7 @@ func (s *Hosts) Delete(ctx context.Context, id int64) error {
 func normalizeMAC(m string) string {
 	return strings.ToLower(strings.TrimSpace(m))
 }
+
+func normalizeHostname(h string) string {
+	return strings.ToLower(strings.TrimSpace(h))
+}