feat: refactor setup handling and enhance asset serving with bootstrap integration

src/types/index.ts

@@ -2,6 +2,9 @@
 export interface Env {
   DB: D1Database;
   NOTIFICATIONS_HUB: DurableObjectNamespace;
+  ASSETS?: {
+    fetch(input: RequestInfo | URL, init?: RequestInit): Promise<Response>;
+  };
   // Prefer R2 when available. Optional to support KV-only deployments.
   ATTACHMENTS?: R2Bucket;
   // Optional fallback for attachment/send file storage (no credit card required).