Scaling a recipe up shrinks it, and scaling it down grows it

This is the kind of bug a user discovers mid-dinner-party. Someone doubles a recipe from 4 servings to 8 expecting twice the ingredients, and instead gets half -- 250g of chicken instead of 1000g. If they trust the app and don't sanity-check the numbers, they end up under-provisioned for guests, or (scaling down) drowning a 2-person meal in 4x the ingredients. It erodes trust in the one feature (scale) whose entire value proposition is "do the arithmetic correctly so the user doesn't have to." The README's own curl walkthrough documents the correct output (500g -> 1000g for 4 -> 8 servings) -- the running code no longer matches its own docs.

PATCH /recipes/:id/scale with { "targetServings": 8 } on a recipe with servings: 4 returns ingredient quantities divided by 2 instead of multiplied by 2.

curl -X POST http://localhost:3001/recipes -H "Content-Type: application/json" -d '{ "title": "Test Recipe", "cookingTimeMinutes": 20, "servings": 4, "ingredients": [{"name": "Chicken breast", "quantity": 500, "unit": "g"}], "steps": [{"stepNumber": 1, "instruction": "Cook it."}] }'

-> 201, note "id"

curl -X PATCH http://localhost:3001/recipes/<id>/scale -H "Content-Type: application/json" -d '{"targetServings": 8}'

Expected: "quantity": 1000

Actual: "quantity": 250

This is a classic "operands swapped" arithmetic bug: the ratio is computed as recipe.servings / targetServings instead of targetServings / recipe.servings. Both expressions type-check, both produce a plausible-looking number, and unless you check the direction of the result (bigger target -> bigger quantities) the bug is invisible from reading the code casually -- you have to actually work through which operand should be on top.

Find the ratio calculation in the scale handler in src/routes/recipes.js. Think through the ratio from first principles: if targetServings is larger than recipe.servings, the ratio must be greater than 1 (quantities grow); if smaller, less than 1 (quantities shrink). Check which of recipe.servings and targetServings needs to be the numerator for that to hold.