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>

wrangler-example.toml

@@ -8,6 +8,11 @@ kv_namespaces = [
   { binding = "EMAIL_STORAGE", id = "REPLACE_WITH_YOUR_KV_NAMESPACE_ID", preview_id = "REPLACE_WITH_YOUR_PREVIEW_KV_NAMESPACE_ID" }
 ]
 
+# Optional: R2 bucket for storing email attachment enclosures
+# r2_buckets = [
+#   { binding = "ATTACHMENT_BUCKET", bucket_name = "REPLACE_WITH_YOUR_BUCKET_NAME", preview_bucket_name = "REPLACE_WITH_YOUR_PREVIEW_BUCKET_NAME" }
+# ]
+
 # Workers Observability (keeps config in sync with dashboard toggle)
 [observability.logs]
 enabled = true
@@ -42,6 +47,11 @@ kv_namespaces = [
   { binding = "EMAIL_STORAGE", id = "REPLACE_WITH_YOUR_KV_NAMESPACE_ID" }
 ]
 
+# Optional: R2 bucket for storing email attachment enclosures
+# r2_buckets = [
+#   { binding = "ATTACHMENT_BUCKET", bucket_name = "REPLACE_WITH_YOUR_BUCKET_NAME" }
+# ]
+
 routes = [
   { pattern = "REPLACE_WITH_YOUR_DOMAIN", custom_domain = true },
   { pattern = "www.REPLACE_WITH_YOUR_DOMAIN", custom_domain = true }