Add POST /recipes/:id/duplicate (clone a recipe)

Recipe authors constantly want to start a new recipe from an existing one -- a "Chicken Stir Fry" variant that swaps the protein, a smaller/larger batch with tweaked steps, a seasonal remix. Today the only way to do that through this API is to GET the original, hand-copy every field (including every ingredient and every step) into a new POST /recipes body, and resubmit. That's tedious and error-prone by hand, and it means any client wanting a "duplicate this recipe" button has to reimplement the copy logic itself instead of the server owning it. A one-call duplicate endpoint is a small addition with an outsized quality-of-life payoff for anyone actually authoring content through this API.

POST /recipes/:id/duplicate is registered as a stub in src/routes/recipes.js (see the bottom of the file, right before module.exports). It compiles and the route exists, but its handler unconditionally throws -- hitting it always returns a 500 with a generic "Internal server error" body, regardless of :id.

cd nodejs/recipe_api npm run dev # or: docker compose up

curl -X POST http://localhost:3001/recipes/1/duplicate

-> 500 { "error": { "message": "Internal server error", ... } }

the stub handler unconditionally throws, for every :id

This is architecturally the same shape as POST /recipes (a nested create of a recipe plus its ingredients and steps in one transaction) -- except the source data comes from an existing DB row instead of the request body. The natural trap is copying the original ingredient/step objects wholesale into the nested create, id/recipeId fields and all -- Prisma's nested create needs plain data (name/quantity/unit, stepNumber/instruction), not rows that already carry a foreign key pointing at a different recipe.

Start from findRecipeOr404 (already imported/available in this file) to fetch the source recipe with its ingredients/steps included. Then look at how POST /recipes's handler builds its prisma.recipe.create({ data: { ..., ingredients: { create: ingredients }, steps: { create: steps } } }) call -- you need the same shape, but the ingredients/steps arrays come from the fetched recipe's relations (mapped down to just the fields a nested create accepts) instead of req.body.