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)

public/locales/de.json

@@ -234,7 +234,10 @@
     "ingredientCount": "{{count}} Zutat",
     "ingredientCountPlural": "{{count}} Zutaten",
     "titleRequired": "Titel ist erforderlich",
-    "loadingIndicator": "Lade…"
+    "loadingIndicator": "Lade…",
+    "recipeUrlLabel": "Rezept-Link (optional)",
+    "recipeUrlPlaceholder": "https://…",
+    "openRecipe": "Rezept öffnen"
   },
 
   "calendar": {