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>

ShareExtension/ShareView.swift

@@ -15,6 +15,7 @@ struct ShareView: View {
     @State private var tagText = ""
     @State private var notes = ""
     @State private var readLater = false
+    @State private var createPodcast = false
     @State private var suggestedTags: [String] = []   // linkding auto_tags
     @State private var knownTags: Set<String> = []     // user's existing vocabulary (lowercased)
     @State private var aiSuggestions: [String] = []    // net-new AI ideas (not in vocabulary)
@@ -151,6 +152,11 @@ struct ShareView: View {
 
             Toggle("Read later", isOn: $readLater)
                 .font(.subheadline)
+
+            Toggle(isOn: $createPodcast) {
+                Label("Create podcast", systemImage: "headphones")
+            }
+            .font(.subheadline)
         }
     }
 
@@ -236,6 +242,8 @@ struct ShareView: View {
                 title = result.metadata?.title ?? ""
                 suggestedTags = result.autoTags ?? []
             }
+            // Pre-enable the toggle when the current tags match the auto-rule.
+            createPodcast = PodcastRequests.matchesAutoTag(parsedTags)
             phase = .editing
             await loadSuggestions()
         } catch {
@@ -294,6 +302,11 @@ struct ShareView: View {
                 )
                 _ = try await api.createBookmark(create)
             }
+            // Hand off podcast generation to the main app (the extension is too
+            // short-lived to generate audio itself).
+            if createPodcast || PodcastRequests.matchesAutoTag(parsedTags) {
+                PodcastRequests.enqueue(url: url, title: title.isEmpty ? url : title)
+            }
             phase = .done
             try? await Task.sleep(nanoseconds: 700_000_000)
             onClose(true)