feat: split random into 'Was koche ich?' + 'Was backe ich?'

- Two dice buttons on homepage
- Cook: mittag, abend, fruehstueck categories
- Bake: backen, torten, desserts categories
- Backend: /api/recipes/random?categories=slug1,slug2 filter
- Luna's feature request

backend/src/routes/recipes.ts

@@ -4,7 +4,9 @@ import * as svc from '../services/recipe.service.js';
 
 export async function recipeRoutes(app: FastifyInstance) {
   app.get('/api/recipes/random', async (request, reply) => {
-    const recipe = await svc.getRandomRecipe();
+    const { categories } = request.query as { categories?: string };
+    const categorySlugs = categories ? categories.split(',') : undefined;
+    const recipe = await svc.getRandomRecipe(categorySlugs);
     if (!recipe) return reply.status(404).send({ error: 'No recipes found' });
     return recipe;
   });

backend/src/services/recipe.service.ts

@@ -261,8 +261,18 @@ export async function toggleFavorite(id: string, userId?: string) {
   }
 }
 
-export async function getRandomRecipe() {
-  const { rows } = await query('SELECT slug FROM recipes ORDER BY RANDOM() LIMIT 1');
+export async function getRandomRecipe(categorySlugs?: string[]) {
+  let sql = 'SELECT r.slug FROM recipes r';
+  const params: string[] = [];
+  
+  if (categorySlugs && categorySlugs.length > 0) {
+    sql += ' JOIN categories c ON r.category_id = c.id WHERE c.slug = ANY($1)';
+    params.push(categorySlugs as any);
+  }
+  
+  sql += ' ORDER BY RANDOM() LIMIT 1';
+  
+  const { rows } = await query(sql, params);
   if (rows.length === 0) return null;
   return getRecipeBySlug(rows[0].slug);
 }