feat(templates): lock ParentTemplateId after creation

Template inheritance is set once at create time and immutable on update.
UpdateTemplateAsync now returns "Parent template cannot be changed after
creation." when the caller sends a parent that differs from the stored
value — server-side enforcement covers UI, ManagementService, and CLI.
TemplateEdit renders the parent as static plaintext rather than an
editable dropdown; TemplateCreate's parent picker is unchanged.

src/ScadaLink.CentralUI/Components/Pages/Design/TemplateEdit.razor

@@ -212,13 +212,11 @@
                     </div>
                     <div class="col-md-3">
                         <label class="form-label small">Parent Template</label>
-                        <select class="form-select form-select-sm" @bind="_editParentId">
-                            <option value="0">(None)</option>
-                            @foreach (var t in _templates.Where(t => t.Id != _selectedTemplate.Id))
-                            {
-                                <option value="@t.Id">@t.Name</option>
-                            }
-                        </select>
+                        <div class="form-control-plaintext form-control-sm">
+                            @(_selectedTemplate.ParentTemplateId is int pid
+                                ? _templates.FirstOrDefault(t => t.Id == pid)?.Name ?? $"#{pid}"
+                                : "(none)")
+                        </div>
                     </div>
                     <div class="col-md-2">
                         <button class="btn btn-primary btn-sm" @onclick="UpdateTemplateProperties">Save Properties</button>