Enforce unique recipe item positions per recipe (Maestro 75bb8cc1) (#198)

Co-authored-by: orchestrator-agent <20+orchestrator-agent@code.cubecraftcreations.com>

migrations/0018_recipe_item_position_unique.sql

@@ -0,0 +1,9 @@
+-- 0018_recipe_item_position_unique: position is the only record of recipe
+-- order, so a recipe never has two ingredients or two steps at one
+-- position. Writers replace a recipe's items whole, numbered from 0, and
+-- the detail read orders by position alone. A database already holding a
+-- duplicate fails this migration rather than keep an order nobody can
+-- trust.
+
+CREATE UNIQUE INDEX recipe_ingredients_recipe_position ON recipe_ingredients (recipe_id, position);
+CREATE UNIQUE INDEX recipe_steps_recipe_position ON recipe_steps (recipe_id, position);