RecipeIngredientChecks

PUT /v1/recipes/{recipe}/ingredient-checks request body. The wire shape is `{ "checks": [{ "recipe_ingredient_id": "...", "checked": true }, ...] }`. Batched for two reasons: 1. The SPA debounces user taps at 400ms (UI-SPEC §13) and coalesces pending checks into a single PUT to keep request rate well under the api throttle (T-11-S11). 2. Atomic per-batch — handler validates EVERY recipe_ingredient_id belongs to the addressed recipe before any upsert. `user_id` is NOT in the body — it always comes from auth context (T-11-S10 mitigation).