check_dependencies read watchdog.__version__ for its log line; recent
watchdog releases have no such attribute, so the AttributeError escaped and
took the whole diagnostics run down. Read it with a default.
Also covers utils/diagnostics.py: database and images-table checks against
a real temporary database, file system check, ComfyUI output detection
(folder_paths, configured directories, relative fallback, nothing found),
dependencies, the full run and the test image link.