fix: Task back button navigates to correct recipe (not version_id)

- Add recipe_id property to RecipeTask model (via version relationship)
- Add recipe_id to TaskResponse schema
- Eager-load version in _get_task_or_404 query
- Use task.recipe_id instead of task.version_id in task_execute template URLs

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

server/schemas/task.py

@@ -73,6 +73,7 @@ class TaskResponse(BaseModel):
 
     id: int
     version_id: int
+    recipe_id: Optional[int] = None
     order_index: int
     title: str
     directive: Optional[str] = None