feat: add SQLite stats database layer for CivitAI stats

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>

conftest.py

@@ -0,0 +1,3 @@
+# Root-level conftest: prevent pytest from collecting the package __init__.py
+# as a test module (it uses relative imports that require ComfyUI context).
+collect_ignore = ["__init__.py"]