feat(attachments): R2 toggle, storage metrics, and demo R2 config

Add an ATTACHMENTS_ENABLED switch (default on when R2 is bound) via a
central getAttachmentBucket helper, surface R2 + estimated KV usage
against the free tier on the status page and /api/stats (refreshed by the
hourly cron), let setup.sh create and wire the R2 bucket, and bind the
demo bucket so the deployed demo has attachments.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/routes/entries.ts

@@ -2,12 +2,7 @@ import { Context } from "hono";
 import { html, raw } from "hono/html";
 import { Env, FeedConfig, FeedMetadata, EmailData } from "../types";
 import { processEmailContent } from "../utils/html-processor";
-
-function formatBytes(bytes: number): string {
-  if (bytes < 1024) return `${bytes} B`;
-  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
-  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
-}
+import { formatBytes } from "../utils/format";
 
 export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
   const feedId = c.req.param("feedId");