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/Info.plist

@@ -24,5 +24,9 @@
     <array>
         <string>UIInterfaceOrientationPortrait</string>
     </array>
+    <key>UIBackgroundModes</key>
+    <array>
+        <string>audio</string>
+    </array>
 </dict>
 </plist>