Podcast: local MP3 cache, AirPods, lock screen controls, background audio

- Cache generated MP3 to App Support/podcasts/<hash>.mp3 — re-opening
  the same article plays instantly with zero backend calls
- AVAudioSession .playback/.spokenAudio + .allowBluetoothA2DP — routes
  audio to AirPods and Bluetooth headsets
- MPRemoteCommandCenter play/pause/seek — AirPods controls + lock screen
- MPNowPlayingInfoCenter — title and scrubber on lock screen
- UIBackgroundModes: audio in Info.plist — playback continues when backgrounded
- Auto-play when player is ready

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Marks/Services/ClaudeService.swift

@@ -67,14 +67,23 @@ struct ClaudeService: Sendable {
         return try JSONDecoder().decode(PodcastStatus.self, from: data)
     }
 
-    func downloadPodcastAudio(jobId: String) async throws -> URL {
+    func downloadPodcastAudio(jobId: String, articleUrl: String) async throws -> URL {
+        let dest = ClaudeService.cachedPodcastURL(for: articleUrl)
+        try FileManager.default.createDirectory(at: dest.deletingLastPathComponent(),
+                                                withIntermediateDirectories: true)
         let data = try await get("/v1/podcast/audio/\(jobId)")
-        let dest = FileManager.default.temporaryDirectory
-            .appendingPathComponent("podcast-\(jobId).mp3")
         try data.write(to: dest, options: .atomic)
         return dest
     }
 
+    static func cachedPodcastURL(for articleUrl: String) -> URL {
+        // djb2 hash — stable, no CryptoKit needed
+        let hash = articleUrl.utf8.reduce(UInt64(5381)) { ($0 &* 31) &+ UInt64($1) }
+        let dir = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask)[0]
+            .appendingPathComponent("podcasts")
+        return dir.appendingPathComponent("\(hash).mp3")
+    }
+
     // MARK: - HTTP primitives
 
     private func post(_ path: String, body: [String: Any]) async throws -> Data {