feat: add support for KV storage mode and enhance attachment handling

src/types/index.ts

@@ -1,7 +1,10 @@
 // Environment bindings
 export interface Env {
   DB: D1Database;
-  ATTACHMENTS: R2Bucket;
+  // Prefer R2 when available. Optional to support KV-only deployments.
+  ATTACHMENTS?: R2Bucket;
+  // Optional fallback for attachment/send file storage (no credit card required).
+  ATTACHMENTS_KV?: KVNamespace;
   JWT_SECRET: string;
   TOTP_SECRET?: string;
 }