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 <noreply@anthropic.com>

src/test/setup.ts

@@ -237,6 +237,14 @@ export class MockR2 {
     }
   }
 
+  async list(_options?: { cursor?: string }) {
+    const objects = Array.from(this.store.entries()).map(([key, entry]) => ({
+      key,
+      size: entry.body.byteLength,
+    }));
+    return { objects, truncated: false as const, cursor: undefined };
+  }
+
   _has(key: string) {
     return this.store.has(key);
   }