fix: use NullPool for db engine under pytest to avoid cross-event-loop asyncpg errors

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.

backend/app/db.py

@@ -1,9 +1,23 @@
+import sys
+
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
 from sqlalchemy.orm import DeclarativeBase
+from sqlalchemy.pool import NullPool
 
 from app.config import settings
 
-engine = create_async_engine(settings.database_url, echo=False)
+# pytest-asyncio gives each test function its own event loop by default, but
+# this engine (and its connection pool) is a module-level singleton created
+# once and reused across every test. asyncpg connections are bound to the
+# event loop they were opened on, so pooling them across per-test loops
+# raises "got Future attached to a different loop". NullPool sidesteps this
+# by opening a fresh connection per checkout instead of reusing one from a
+# prior loop. Only applied under pytest; production keeps normal pooling.
+_engine_kwargs = {"echo": False}
+if "pytest" in sys.modules:
+    _engine_kwargs["poolclass"] = NullPool
+
+engine = create_async_engine(settings.database_url, **_engine_kwargs)
 async_session_maker = async_sessionmaker(engine, expire_on_commit=False)