feat: per-ingredient category selection for shopping list transfer (closes #33)

When adding ingredients in the meal editor, each ingredient now has a
category dropdown. Categories are stored on the ingredient and applied
automatically when transferring to the shopping list, so items appear
pre-grouped by category without manual re-sorting.

server/db.js

@@ -362,6 +362,13 @@ const MIGRATIONS = [
       ALTER TABLE meals ADD COLUMN recipe_url TEXT;
     `,
   },
+  {
+    version: 7,
+    description: 'Kategorie pro Zutat für Einkaufslisten-Transfer',
+    up: `
+      ALTER TABLE meal_ingredients ADD COLUMN category TEXT NOT NULL DEFAULT 'Sonstiges';
+    `,
+  },
 ];
 
 /**