fix(todos): datetime query params typés FastAPI, ORDER BY déterministe

- Changer due_after/due_before de str | None vers datetime | None pour typage FastAPI
- FastAPI parse et valide automatiquement, retourne 422 si format invalide (pas 500)
- Supprimer le parsing manuel datetime.fromisoformat() qui levait ValueError brute
- Ajouter ORDER BY déterministe: due_date ASC NULLS LAST, created_at DESC
  évite les réordonnances aléatoires entre requêtes PostgreSQL

Tests: 15/15 passent ✓

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

backend/app/api/todos.py

@@ -18,8 +18,8 @@ async def list_todos(
     status: str | None = "pending",
     priority: str | None = None,
     tag: str | None = None,
-    due_after: str | None = None,
+    due_after: datetime | None = None,
-    due_before: str | None = None,
+    due_before: datetime | None = None,
     limit: int = 200,
     session: AsyncSession = Depends(get_session),
 ):
@@ -33,13 +33,14 @@ async def list_todos(
     if tag:
         conditions.append(TodoItem.tags.contains([tag]))
     if due_after:
-        conditions.append(TodoItem.due_date >= datetime.fromisoformat(due_after))
+        conditions.append(TodoItem.due_date >= due_after)
     if due_before:
-        conditions.append(TodoItem.due_date <= datetime.fromisoformat(due_before))
+        conditions.append(TodoItem.due_date <= due_before)
 
     stmt = select(TodoItem)
     if conditions:
         stmt = stmt.where(and_(*conditions))
+    stmt = stmt.order_by(TodoItem.due_date.asc().nulls_last(), TodoItem.created_at.desc())
     stmt = stmt.limit(limit)
     result = await session.execute(stmt)
     return result.scalars().all()