The smoke test used `os.environ.setdefault()` to point at the frozen
fixture, which silently skipped when a prior test in the suite had
already set the env var. Combined with `core.config` caching `DB_PATH`
at module import time, this left smoke tests pointed at the wrong DB
once Wave 1's services tests pre-imported `main` with their own temp
state.
Exposed by Wave 2's additional tests (PR #94) pushing collection order
past the tipping point, but the underlying pollution existed since Wave
1 merged — Wave 3 CI passed only by collection-order luck.
Fix mirrors the `sys.modules` purge pattern that
`tests/backend/services/conftest.py` already uses.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>