Add download option and CD disc artwork to finished books; v2.0.31

- Add downloadUrl field to FinishedBook type
- Admin: paste any external URL (Drive, Dropbox, etc.) per finished book
- Public series page: replace square hero image with circular CD disc artwork
  (conic-gradient sheen, center hole, drop shadow, slight tilt)
- Stream Episodes + Download Album action buttons in the hero
- Grid cards show a gold download badge when a download URL is set

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/content.ts

@@ -91,6 +91,7 @@ export interface FinishedBook {
   description: string
   imageUrl: string
   season: number
+  downloadUrl?: string
 }
 
 export interface PodcastHost {