fix(attachments): render inline cid: images in emails and feeds

Capture each attachment's Content-ID at ingestion (postal-mime and
mailparser paths) and rewrite cid: image refs to the stored /files URL
in processEmailContent, shared by the entry view and RSS/Atom feeds.
Bodyless HTML fragments are now serialized so sanitization and the cid
rewrite apply to them too.

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

src/lib/email-processor.ts

@@ -21,6 +21,7 @@ export interface RawAttachment {
   filename: string;
   contentType: string;
   content: ArrayBuffer;
+  contentId?: string;
 }
 
 export interface ProcessEmailInput {
@@ -88,6 +89,7 @@ async function uploadAttachments(
         filename: att.filename,
         contentType: att.contentType,
         size: att.content.byteLength,
+        ...(att.contentId ? { contentId: att.contentId } : {}),
       };
     }),
   );