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/controller/node.py

@@ -162,6 +162,10 @@ class Node:
     def status(self):
         return self._status
 
+    @property
+    def template_id(self):
+        return self._template_id
+
     @property
     def name(self):
         return self._name