refactor: update Elysia initialization with API prefix and fix API destination in Next.js config

feat: add targetId for Pixiv and Twitter uploads in post routes
style: adjust filter container margin and button styles for improved layout

apps/backend/src/index.ts

@@ -5,7 +5,7 @@ import openapi from "@elysiajs/openapi";
 
 await mongoose.connect(config.mongodb.uri);
 
-const app = new Elysia()
+const app = new Elysia({prefix: "/api"})
   .use(openapi())
   .get("/", () => "어...")