fix(tasks): eager-load version in reorder endpoint to fix MissingGreenlet

The reorder_tasks endpoint loaded RecipeTask with selectinload(subtasks)
only. Serializing via TaskResponse reads RecipeTask.recipe_id, a @property
that traverses the version relationship, triggering a lazy load outside the
async greenlet context -> MissingGreenlet error.

Add selectinload(RecipeTask.version), matching the pattern already used in
the get-task flow. Fixes the previously order-dependent test_reorder_tasks
failure; full suite now 179 passed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/backend/api/routers/tasks.py

@@ -223,7 +223,10 @@ async def reorder_tasks(
     result = await db.execute(
         select(RecipeTask)
         .where(RecipeTask.id.in_(data.task_ids))
-        .options(selectinload(RecipeTask.subtasks))
+        .options(
+            selectinload(RecipeTask.subtasks),
+            selectinload(RecipeTask.version),
+        )
     )
     tasks_map = {t.id: t for t in result.scalars().all()}