feat(attachments): render inline cid images in place, not as attachments

Inline images (referenced by src="cid:…") are now classified at ingest and
kept out of the downloadable attachment lists, RSS/Atom enclosures, and the
API — while still stored in R2 and cleaned up with the email. Fixes the admin
email preview, which injected raw HTML into the data: iframe so cid refs never
resolved; it now rewrites them to absolute /files URLs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/infrastructure/feed-generator.ts

@@ -54,7 +54,8 @@ function buildFeed(
 
   for (const email of emails) {
     const entryUrl = `${baseUrl}/entries/${feedId}/${email.receivedAt}`;
-    const firstAttachment = email.attachments?.[0];
+    // Inline images are rendered in the body, not surfaced as an enclosure.
+    const firstAttachment = email.attachments?.find((a) => !a.inline);
     const bodyContent = processEmailContent(
       email.content,
       email.attachments,