feat: forbid deleting templates and images still used by projects

Deleting a template with prune_images, deleting an image, or pruning
orphan images only checked template references — a project node still
pointing at the image (e.g. via hda_disk_image_backing_file) was left
with a dangling reference and the project could no longer be opened.

- Add controller helpers scanning every known project (opened projects
  via in-memory nodes, closed projects via their .gns3 file) for
  template and image usage; unreadable topologies are skipped
- Guard DELETE /templates/{id}, DELETE /templates/{id}?prune_images,
  DELETE /images/{path} and /images/prune with a 409 listing the
  project names
- Run all template-delete checks before any mutation so a refused
  deletion cannot leave the template gone while its images survive

gns3server/services/templates.py

@@ -341,6 +341,15 @@ class TemplatesService:
 
         if self.get_builtin_template(template_id):
             raise ControllerForbiddenError(f"Template '{template_id}' cannot be deleted because it is built-in")
+
+        template = await self.get_template(template_id)
+        project_names = self._controller.find_projects_using_template(template_id)
+        if project_names:
+            raise ControllerError(
+                f"Template '{template['name']}' cannot be deleted because it is used by "
+                f"one or more projects: {', '.join(project_names)}"
+            )
+
         if await self._templates_repo.delete_template(template_id):
             self._controller.notification.controller_emit("template.deleted", {"template_id": str(template_id)})
         else: