feat(podcasts): played/unplayed queue, background generation, share-sheet audio

Turns the Podcasts tab into a proper podcast-app experience across three
features that compose on a shared background-generation service.

Background generation (#2)
- New PodcastGenerationManager runs generate→poll→download off the player,
  so producing a new episode never stops current playback.
- Headphone taps never interrupt: idle → generate-and-play in the foreground
  (unchanged); already playing → generate in the background, episode lands in
  the library. A "Generating…" banner surfaces active jobs.

Played/unplayed + queue (#1)
- PodcastEntry gains playedAt (old index.json decodes as unplayed).
- Podcasts tab splits into Up Next / Played with a Play All queue that
  auto-advances; finishing marks played; swipe to toggle played state.

Share-extension audio (#3)
- "Create podcast" toggle on the save card, plus configurable Auto-Podcast
  Tags in Settings. The extension enqueues a request into the App Group; the
  main app drains it on launch/foreground and generates in the background.

Closes #1
Closes #2
Closes #3

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

project.yml

@@ -90,6 +90,8 @@ targets:
       - path: Marks/Services/LinkdingAPI.swift
       - path: Marks/Services/MarksAuth.swift
       - path: Marks/Services/Log.swift
+      # Cross-process podcast request queue: the extension enqueues, the app runs it.
+      - path: Marks/Services/PodcastRequests.swift
     settings:
       base:
         PRODUCT_BUNDLE_IDENTIFIER: com.magicive.marks.ShareExtension