feat(todos): endpoints CRUD + postpone — 15 tests passent

Ajoute les 5 endpoints REST (list, create, update, delete, postpone),
enregistre le routeur sur /api/todos, et corrige l'isolation des sessions
de test via NullPool + dependency_overrides dans conftest.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/conftest.py

@@ -1,16 +1,34 @@
 import pytest
 from httpx import AsyncClient, ASGITransport
+from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker, AsyncSession
+from sqlalchemy.pool import NullPool
 from app.main import app
-from app.core.database import AsyncSessionLocal
+from app.core import database
+from app.core.config import settings
 
 
-@pytest.fixture
-async def client():
-    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as ac:
-        yield ac
+def make_test_engine():
+    """Crée un engine sans pool pour éviter les conflits d'event loop entre tests."""
+    return create_async_engine(settings.database_url, poolclass=NullPool)
 
 
 @pytest.fixture
 async def db_session():
-    async with AsyncSessionLocal() as session:
+    engine = make_test_engine()
+    session_factory = async_sessionmaker(engine, expire_on_commit=False)
+    async with session_factory() as session:
         yield session
+    await engine.dispose()
+
+
+@pytest.fixture
+async def client(db_session: AsyncSession):
+    """Client HTTP qui injecte la session de test dans l'app FastAPI."""
+
+    async def override_get_session():
+        yield db_session
+
+    app.dependency_overrides[database.get_session] = override_get_session
+    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as ac:
+        yield ac
+    app.dependency_overrides.clear()