Implements check_folder_quota and add_token_usage in app/billing/quota.py
with dialect-aware upsert (pg_insert on PostgreSQL, read-then-write on SQLite).
Adds test_user_free/test_user_power fixtures and db alias to conftest.py.
6 new tests pass.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
