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/lib/services/site-settings.ts

@@ -1,12 +1,11 @@
 import "server-only";
 
-import { FALLBACK_HOTEL_NAME } from "@/lib/brand";
 import { db, WebsiteSetting } from "@/lib/db";
 import { logger } from "@/lib/logger";
 import { redis } from "@/lib/redis";
 
 const DEFAULTS: Record<string, string> = {
-	hotel_name: FALLBACK_HOTEL_NAME,
+	hotel_name: "",
 	habbo_imaging_url: "/imaging",
 	logo_url: "",
 	nitro_client_url: "",