feat: add Web Vault visibility switch

src/types/index.ts

@@ -6,6 +6,8 @@ export interface Env {
   ASSETS?: {
     fetch(input: RequestInfo | URL, init?: RequestInit): Promise<Response>;
   };
+  // Set to "1" to return 404 for the Web Vault while keeping client APIs available.
+  HIDE_WEB_VAULT?: string;
   // Prefer R2 when available. Optional to support KV-only deployments.
   ATTACHMENTS?: R2Bucket;
   // Optional fallback for attachment/send file storage (no credit card required).