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/api/routes/controller/images.py

@@ -197,11 +197,16 @@ async def prune_images(
     """
     Prune images not attached to any template.
 
+    Images referenced by a node in any project (opened or closed) are kept.
+
     Required privilege: Image.Allocate
     """
 
     skip_images = get_builtin_disks()
-    await images_repo.prune_images(skip_images)
+    # a single pass over all projects' node properties protects every
+    # referenced file name at once
+    referenced_filenames = Controller.instance().collect_referenced_image_filenames()
+    await images_repo.prune_images(list(skip_images) + list(referenced_filenames))
 
 
 @router.post(
@@ -309,6 +314,10 @@ async def delete_image(
         template_names = ", ".join([template.name for template in templates])
         raise ControllerError(f"Image '{image_path}' is used by one or more templates: {template_names}")
 
+    project_names = Controller.instance().find_projects_using_image(image.filename)
+    if project_names:
+        raise ControllerError(f"Image '{image_path}' is used by one or more projects: {', '.join(project_names)}")
+
     try:
         os.remove(image.path)
     except OSError:

gns3server/api/routes/controller/templates.py

@@ -35,6 +35,7 @@ from gns3server.db.repositories.templates import TemplatesRepository
 from gns3server.services.templates import TemplatesService
 from gns3server.db.repositories.rbac import RbacRepository
 from gns3server.db.repositories.images import ImagesRepository
+from gns3server.controller import Controller
 from gns3server.controller.controller_error import ControllerError, ControllerBadRequestError
 from gns3server.utils.images import get_builtin_disks
 
@@ -135,28 +136,49 @@ async def delete_template(
     Required privilege: Template.Allocate
     """
 
+    controller = Controller.instance()
     images = await templates_repo.get_template_images(template_id)
-    await TemplatesService(templates_repo).delete_template(template_id)
-    await rbac_repo.delete_all_ace_starting_with_path(f"/templates/{template_id}")
+
+    # Run every usage check before mutating anything, so a refused deletion
+    # cannot leave the template gone while its images survive.
+    images_to_prune = []
     if prune_images and images:
         skip_images = get_builtin_disks()
+        # collected lazily: a single pass over all projects' node properties
+        referenced_filenames = None
         for image in images:
             if image.filename in skip_images:
                 continue
-            templates = await images_repo.get_image_templates(image.image_id)
-            if templates:
-                template_names = ", ".join([template.name for template in templates])
+            # the template being deleted is still in the database at this
+            # point, exclude it from the other-templates check
+            other_templates = [
+                template for template in await images_repo.get_image_templates(image.image_id)
+                if str(template.template_id) != str(template_id)
+            ]
+            if other_templates:
+                template_names = ", ".join([template.name for template in other_templates])
                 raise ControllerError(f"Image '{image.path}' is used by one or more templates: {template_names}")
 
-            try:
-                os.remove(image.path)
-            except OSError:
-                log.warning(f"Could not delete image file {image.path}")
+            if referenced_filenames is None:
+                referenced_filenames = controller.collect_referenced_image_filenames()
+            if image.filename in referenced_filenames:
+                project_names = controller.find_projects_using_image(image.filename)
+                raise ControllerError(f"Image '{image.path}' is used by one or more projects: {', '.join(project_names)}")
+            images_to_prune.append(image)
 
-            print(f"Deleting image '{image.path}'")
-            success = await images_repo.delete_image(image.path)
-            if not success:
-                raise ControllerError(f"Image '{image.path}' could not removed from the database")
+    await TemplatesService(templates_repo).delete_template(template_id)
+    await rbac_repo.delete_all_ace_starting_with_path(f"/templates/{template_id}")
+
+    for image in images_to_prune:
+        try:
+            os.remove(image.path)
+        except OSError:
+            log.warning(f"Could not delete image file {image.path}")
+
+        log.info(f"Deleting image '{image.path}'")
+        success = await images_repo.delete_image(image.path)
+        if not success:
+            raise ControllerError(f"Image '{image.path}' could not removed from the database")
 
 
 @router.get(