test: improve canvas route coverage to 100% and fix async tracing

- Add 22 new tests for canvas save/load: properties round-trip,
  edge waypoints, edge handles, animated, parent/child nodes,
  group nodes, all node fields, bottom_handles, services/notes,
  check_method/target, viewport update branches, edge update branch
- Fix coverage.py async tracing by setting core = "sysmon" in
  pyproject.toml (Python 3.12+ sys.monitoring API), resolving the
  sys.settrace drop-after-await issue

backend/pyproject.toml

@@ -25,6 +25,8 @@ addopts = "--tb=short -q"
 [tool.coverage.run]
 source = ["app"]
 omit = ["*/migrations/*", "*/tests/*"]
+concurrency = ["thread"]
+core = "sysmon"
 
 [tool.coverage.report]
 skip_empty = true