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/app/main.py

@@ -3,6 +3,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 from app.api.health import router as health_router
 from app.api.media import router as media_router
+from app.api.todos import router as todos_router
 from app.core.config import settings
 from app.data.seed import run_seed
 
@@ -25,3 +26,4 @@ app.add_middleware(
 
 app.include_router(health_router, prefix="/api")
 app.include_router(media_router, prefix="/api/media")
+app.include_router(todos_router, prefix="/api/todos")