Implements StatsDB class with upsert, batch upsert, get_by_hashes,
get_all, count, and close methods backed by a WAL-mode SQLite database.
Also guards __init__.py relative imports behind __package__ check so
pytest can run without ComfyUI context, and removes empty tests/__init__.py
to prevent pytest Package collector from traversing into the plugin root.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
