Production code shouldn't branch on `"pytest" in sys.modules` — it's
fragile and couples db.py to test tooling. Instead, conftest.py now
builds its own dedicated NullPool engine directly from settings, used
only for the drop_all/create_all reset and the get_db override. The
production engine in app/db.py is untouched and never exercised during
tests, so this fully preserves the event-loop fix while keeping prod
code test-agnostic.
Task 3's conftest.py (per plan) reuses the module-level app.db.engine
singleton across every test. pytest-asyncio 0.24 gives each test function
its own event loop by default, and asyncpg connections are bound to the
loop they were opened on. Pooling a connection from a prior test's loop
made subsequent tests fail with "got Future attached to a different loop"
as soon as more than one DB-touching test ran in the same session.
NullPool is applied only when running under pytest (detected via
sys.modules), so production keeps normal connection pooling and only the
test suite pays the cost of a fresh connection per checkout.
