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 <noreply@anthropic.com>

src/types/index.ts

@@ -18,6 +18,7 @@ export interface AttachmentData {
   filename: string;
   contentType: string;
   size: number;
+  contentId?: string; // Normalized Content-ID (no <>) used to resolve inline cid: refs
 }
 
 // Email interface for stored emails