feat(catalog/sync): Promote pure RSS feeds to catalog parity (background refresh, notifications, auto & smart downloads)

[ashwkun]

Overview & Background

Subscribed podcasts imported directly via custom RSS URLs (or OPML) are currently treated as second-class citizens compared to catalog shows, refreshing mostly on-demand. Since the app fetches subscribed feeds directly from their source RSS endpoints, pure RSS feeds should have full parity with catalog podcasts.

Proposed Scope

1. Periodic Background Synchronization:
   • Incorporate subscribed pure RSS feeds into the background WorkManager refresh cadence alongside catalog shows.
   • Respect conditional HTTP headers (ETag, If-Modified-Since) to minimize unnecessary data usage and battery drain.
2. New Episode Notifications:
   • Detect newly published episodes during background sync for pure RSS feeds.
   • Dispatch notifications based on the user's notification preferences for that show.
3. Automatic & Smart Downloads:
   • Enable per-show "Auto Download" toggles for pure RSS feeds (e.g., download newest 1/3/5 episodes).
   • Integrate pure RSS episodes into SmartDownloadManager queue refill logic.
4. Identity & Storage Contracts:
   • Maintain strict backward compatibility for rss: URI prefixes and negative ID mappings in Room.
   • Ensure episodic supplements and episode GUIDs remain stable across feed refreshes.

Acceptance Criteria

• Subscribed pure RSS feeds are refreshed periodically in background workers without requiring manual on-demand pull-to-refresh.
• Users can enable new episode notifications for pure RSS shows.
• Auto-download and Smart Downloads support pure RSS episodes according to user storage rules.
• Preserves all existing identity contracts (rss: IDs, negative ID conventions).
• JVM unit tests verify sync scheduling and episode insertion logic for pure RSS URLs.