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>

internal/db/recipes.sql.go

@@ -220,7 +220,7 @@ func (q *Queries) GetRecipeBySourceURL(ctx context.Context, sourceUrl string) (R
 const listRecipeIngredients = `-- name: ListRecipeIngredients :many
 SELECT id, recipe_id, position, raw_text, name, qty, unit, note, created_at, updated_at FROM recipe_ingredients
 WHERE recipe_id = ?
-ORDER BY position, id
+ORDER BY position
 `
 
 func (q *Queries) ListRecipeIngredients(ctx context.Context, recipeID string) ([]RecipeIngredient, error) {
@@ -260,7 +260,7 @@ func (q *Queries) ListRecipeIngredients(ctx context.Context, recipeID string) ([
 const listRecipeSteps = `-- name: ListRecipeSteps :many
 SELECT id, recipe_id, position, text, created_at, updated_at FROM recipe_steps
 WHERE recipe_id = ?
-ORDER BY position, id
+ORDER BY position
 `
 
 func (q *Queries) ListRecipeSteps(ctx context.Context, recipeID string) ([]RecipeStep, error) {