feat(meals): add optional recipe link to meal cards (#18)

- New optional recipe_url field in the meal modal (below Notes)
- Link icon appears on meal cards when a URL is set, opens in new tab
- DB migration v6: ALTER TABLE meals ADD COLUMN recipe_url TEXT
- API: recipe_url supported in POST /meals and PUT /meals/:id
- i18n: new keys recipeUrlLabel, recipeUrlPlaceholder, openRecipe (de, en, sv, it)

server/db.js

@@ -355,6 +355,13 @@ const MIGRATIONS = [
         ('Sonstiges',        'shopping-basket', 8);
     `,
   },
+  {
+    version: 6,
+    description: 'Rezept-URL für Mahlzeiten',
+    up: `
+      ALTER TABLE meals ADD COLUMN recipe_url TEXT;
+    `,
+  },
 ];
 
 /**