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>

Marks/Services/PodcastIndex.swift

@@ -7,6 +7,11 @@ struct PodcastEntry: Codable, Identifiable {
     var title: String?
     let createdAt: Date
     var parentBookmarkUrl: String?
+    /// When the episode was finished. `nil` means unplayed/new. Absent in
+    /// pre-existing index files, which decode as `nil` (i.e. unplayed).
+    var playedAt: Date? = nil
+
+    var isPlayed: Bool { playedAt != nil }
 }
 
 enum PodcastIndex {
@@ -49,6 +54,14 @@ enum PodcastIndex {
         all().filter { $0.parentBookmarkUrl == bookmarkUrl || $0.articleUrl == bookmarkUrl }
     }
 
+    /// Mark an episode played (default) or back to unplayed. No-op if unknown.
+    static func setPlayed(articleUrl: String, _ played: Bool = true) {
+        var entries = all()
+        guard let i = entries.firstIndex(where: { $0.articleUrl == articleUrl }) else { return }
+        entries[i].playedAt = played ? Date() : nil
+        save(entries)
+    }
+
     static func remove(articleUrl: String) {
         var entries = all()
         entries.removeAll { $0.articleUrl == articleUrl }