Also fixes async-test DB isolation: test_mcp_toolkit.py drives real MCP
dispatch, which runs tool functions' ORM calls on a sync_to_async worker
thread outside pytest-django's per-test atomic transaction, so writes
committed there weren't rolled back and leaked into later tests. Sets
DJANGO_ALLOW_ASYNC_UNSAFE for the async ORM setup calls and flushes the
Printer table after this file's tests via conftest.py.
