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/types/index.ts

@@ -19,6 +19,10 @@ export interface AttachmentData {
   contentType: string;
   size: number;
   contentId?: string; // Normalized Content-ID (no <>) used to resolve inline cid: refs
+  // True when this attachment is an inline image referenced by a cid: URL in the
+  // email body. Inline attachments render in place and are hidden from the
+  // downloadable attachment lists, but are still stored in R2 and cleaned up.
+  inline?: boolean;
 }
 
 // Email interface for stored emails
@@ -59,7 +63,8 @@ export interface EmailMetadata {
   subject: string;
   receivedAt: number;
   size?: number;
-  attachmentIds?: string[];
+  attachmentIds?: string[]; // Downloadable attachments (shown to the user)
+  inlineAttachmentIds?: string[]; // Inline images: hidden from lists, still cleaned up
 }
 
 // Feed list interface