Enforce unique recipe item positions per recipe

Add migration 0018 with unique indexes on recipe_ingredients and
recipe_steps (recipe_id, position), so a recipe can never hold two
ingredients or two steps at one position. The dinner detail read now
orders by position alone and drops its read-time duplicate check; a
database already holding a duplicate fails the migration.

Maestro task 75bb8cc1-fd30-4050-90d2-a1627a865b22

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/queries/recipes.sql

@@ -48,7 +48,7 @@ INSERT INTO recipe_ingredients (
 -- name: ListRecipeIngredients :many
 SELECT * FROM recipe_ingredients
 WHERE recipe_id = ?
-ORDER BY position, id;
+ORDER BY position;
 
 -- name: DeleteRecipeIngredients :exec
 DELETE FROM recipe_ingredients
@@ -64,7 +64,7 @@ INSERT INTO recipe_steps (
 -- name: ListRecipeSteps :many
 SELECT * FROM recipe_steps
 WHERE recipe_id = ?
-ORDER BY position, id;
+ORDER BY position;
 
 -- name: DeleteRecipeSteps :exec
 DELETE FROM recipe_steps