refactor: remove hotel-name fallback, fail fast when unconfigured

Drop the hardcoded FALLBACK_HOTEL_NAME ("Atom") preset and the brand.ts
module. HOTEL_NAME is now a required env var: if it (and the CMS hotel_name
setting) is missing the site fails validation at startup/build with a clear
message instead of silently rendering a placeholder hotel name.

resolveHotelName() resolves CMS hotel_name -> required HOTEL_NAME only.
Callers that used the preset (api/home route catch branch, CMS settings form
default, mobile-nav/logo-generator prop defaults) now use the configured name
or an empty default; the real name is already passed in by server parents.

src/app/api/home/route.ts

@@ -1,6 +1,6 @@
 import { count, desc, eq } from "drizzle-orm";
+import { env } from "@/env";
 import { apiJson } from "@/lib/api";
-import { FALLBACK_HOTEL_NAME } from "@/lib/brand";
 import { db, User, WebsiteArticles } from "@/lib/db";
 import { resolveHotelName } from "@/lib/hotel-name";
 import { apiCacheKey, cacheSafe, redisCache } from "@/lib/redis-cache";
@@ -38,7 +38,7 @@ export async function GET(_req: Request) {
 		return apiJson(data);
 	} catch {
 		return apiJson(
-			{ articles: [], online: 0, hotelName: FALLBACK_HOTEL_NAME },
+			{ articles: [], online: 0, hotelName: env.HOTEL_NAME },
 			{ status: 200 },
 		);
 	}