Straw phase M-2: MediaSession + audio focus + service skeleton

Wraps the existing Activity-owned ExoPlayer in a Media3 MediaSession. Side
effects:
- Lock-screen media controls (play/pause)
- System media notification with play/pause buttons
- Audio focus + ducking (other audio dims when straw plays)
- Bluetooth + headset hardware-button routing
- Plays nicely alongside other media apps' notifications

The ExoPlayer is also configured with AudioAttributes (USAGE_MEDIA +
CONTENT_TYPE_MOVIE) and handleAudioFocus=true so other apps can request
focus correctly (e.g., a phone call ducks/pauses straw).

Also scaffolded but NOT yet wired:
- PlaybackService extending MediaSessionService — the foundation for
  true background-after-Activity-kill audio. Manifest entry + deps
  added (media3-session 1.4.1, concurrent-futures-ktx 1.2.0). The
  Activity-to-Service migration via MediaController is M-3 work.
- POST_NOTIFICATIONS, FOREGROUND_SERVICE,
  FOREGROUND_SERVICE_MEDIA_PLAYBACK, WAKE_LOCK permissions in manifest.

For the user right now: lock the phone while a video plays — media
controls appear on the lock screen. Open another app — notification with
play/pause sits in the shade. Press a headset's play/pause — straw
responds. Pull the home screen — PiP keeps the video floating + audio
continues. Full screen-off-background-audio after killing the activity
arrives in M-3.

strawApp/src/main/AndroidManifest.xml

@@ -3,6 +3,13 @@
 
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+    <!-- Background audio playback -->
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE_MEDIA_PLAYBACK" />
+    <!-- Android 13+ runtime notification permission for media controls -->
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
+    <!-- Wake while audio plays -->
+    <uses-permission android:name="android.permission.WAKE_LOCK" />
 
     <application
         android:name=".StrawApp"
@@ -39,5 +46,15 @@
                 <data android:mimeType="text/plain" />
             </intent-filter>
         </activity>
+
+        <!-- Phase M-2: MediaSessionService for background audio + notification + lock-screen controls. -->
+        <service
+            android:name=".feature.player.PlaybackService"
+            android:exported="true"
+            android:foregroundServiceType="mediaPlayback">
+            <intent-filter>
+                <action android:name="androidx.media3.session.MediaSessionService" />
+            </intent-filter>
+        </service>
     </application>
 </manifest>