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/Views/SettingsView.swift

@@ -6,6 +6,8 @@ struct SettingsView: View {
 
     @Environment(\.dismiss) private var dismiss
 
+    @State private var autoTagText = PodcastRequests.autoTags.joined(separator: " ")
+
     var body: some View {
         NavigationStack {
             List {
@@ -15,6 +17,21 @@ struct SettingsView: View {
                         .foregroundStyle(.secondary)
                 }
 
+                Section {
+                    TextField("e.g. listen podcast", text: $autoTagText)
+                        .textInputAutocapitalization(.never)
+                        .autocorrectionDisabled()
+                        .onChange(of: autoTagText) { _, new in
+                            PodcastRequests.autoTags = new
+                                .split(whereSeparator: { $0 == " " || $0 == "," })
+                                .map(String.init)
+                        }
+                } header: {
+                    Text("Auto-Podcast Tags")
+                } footer: {
+                    Text("Saving a bookmark with any of these tags automatically generates a podcast for it.")
+                }
+
                 Section("Server") {
                     Button(role: .destructive) {
                         dismiss()