feat(flutter/player): MinstrelAudioHandler + just_audio + audio_service

audio_service runs the handler in a background isolate; UI watches
playbackState/mediaItem/queue streams through Riverpod. AndroidManifest
gets the foreground-service media-playback permission; Info.plist gets
UIBackgroundModes=audio. Bearer token attached to stream URLs via
just_audio's per-source headers.

flutter_client/android/app/src/main/AndroidManifest.xml

@@ -1,4 +1,8 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
+    <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.WAKE_LOCK" />
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE_MEDIA_PLAYBACK" />
     <application
         android:label="minstrel"
         android:name="${applicationName}"
@@ -30,6 +34,21 @@
         <meta-data
             android:name="flutterEmbedding"
             android:value="2" />
+        <service
+            android:name="com.ryanheise.audioservice.AudioService"
+            android:foregroundServiceType="mediaPlayback"
+            android:exported="true">
+            <intent-filter>
+                <action android:name="android.media.browse.MediaBrowserService" />
+            </intent-filter>
+        </service>
+        <receiver
+            android:name="com.ryanheise.audioservice.MediaButtonReceiver"
+            android:exported="true">
+            <intent-filter>
+                <action android:name="android.intent.action.MEDIA_BUTTON" />
+            </intent-filter>
+        </receiver>
     </application>
     <!-- Required to query activities that can process text, see:
          https://developer.android.com/training/package-visibility and