fix: move NullPool test fix from db.py into a conftest-local test engine

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.

backend/tests/conftest.py

@@ -1,16 +1,24 @@
 import pytest_asyncio
 from httpx import ASGITransport, AsyncClient
-from sqlalchemy.ext.asyncio import async_sessionmaker
+from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
+from sqlalchemy.pool import NullPool
 
-from app.db import Base, engine, get_db
+from app.config import settings
+from app.db import Base, get_db
 from app.main import app
 
-TestSessionLocal = async_sessionmaker(engine, expire_on_commit=False)
+# pytest-asyncio gives each test function its own event loop by default;
+# asyncpg connections are bound to the loop they were opened on, so a pooled
+# connection from one test's loop breaks the next test. NullPool sidesteps
+# this by opening a fresh connection per checkout — scoped to this dedicated
+# test engine so app.db.engine (used by production) is unaffected.
+test_engine = create_async_engine(settings.database_url, poolclass=NullPool)
+TestSessionLocal = async_sessionmaker(test_engine, expire_on_commit=False)
 
 
 @pytest_asyncio.fixture(autouse=True)
 async def _reset_db():
-    async with engine.begin() as conn:
+    async with test_engine.begin() as conn:
         await conn.run_sync(Base.metadata.drop_all)
         await conn.run_sync(Base.metadata.create_all)
     yield