feat: store email attachments in R2 and expose as RSS enclosures

Attachments from incoming emails are uploaded to an optional Cloudflare R2
bucket and exposed as <enclosure> elements in RSS and <link rel="enclosure">
in Atom feeds, served at /files/{id}/{filename} with immutable caching.

R2 is opt-in: if ATTACHMENT_BUCKET is not bound the feature is a no-op.
Attachments are cleaned up from R2 on email/feed deletion and during
size-based feed trimming. Adds MockR2 to the test setup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/routes/files.ts

@@ -0,0 +1,33 @@
+import { Context } from "hono";
+import { Env } from "../types";
+
+export async function handle(c: Context): Promise<Response> {
+  const env = c.env as unknown as Env;
+
+  if (!env.ATTACHMENT_BUCKET) {
+    return new Response("Attachment storage not configured", { status: 404 });
+  }
+
+  const attachmentId = c.req.param("attachmentId");
+  const filename = c.req.param("filename");
+
+  const object = await env.ATTACHMENT_BUCKET.get(attachmentId);
+
+  if (!object) {
+    return new Response("Not found", { status: 404 });
+  }
+
+  const headers = new Headers();
+  object.writeHttpMetadata(headers);
+  headers.set("etag", object.httpEtag);
+  headers.set("Cache-Control", "public, max-age=31536000, immutable");
+
+  if (!headers.get("Content-Disposition")) {
+    headers.set(
+      "Content-Disposition",
+      `attachment; filename="${decodeURIComponent(filename)}"`,
+    );
+  }
+
+  return new Response(object.body, { headers });
+}