feat: let ExoPlayer handle audio focus (pause on calls / other media apps)

The shared feed/podcast/music ExoPlayer previously set no audio attributes and
did not handle audio focus, so a phone call or another media app starting would
not pause playback — meaning V4V streaming payments could keep accruing during a
call even though the user wasn't really listening.

Set USAGE_MEDIA audio attributes with handleAudioFocus = true on the pooled
player. ExoPlayer now pauses on focus loss (a call, another app's playback) and
ducks for transient interruptions; pausing flips isPlaying false, so the
streaming-payment accrual stops with it for free.

Tradeoff: in Media3 a muted player still requests focus while playWhenReady is
true, so muted feed autoplay now requests audio focus too. Acceptable for
correct call/interruption behavior; can be scoped to audio-only players later if
it proves disruptive.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JGa1EM5KWyDo1o5Yr6sS18

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/playerPool/ExoPlayerBuilder.kt

@@ -22,6 +22,8 @@ package com.vitorpamplona.amethyst.service.playback.playerPool
 
 import android.content.Context
 import androidx.annotation.OptIn
+import androidx.media3.common.AudioAttributes
+import androidx.media3.common.C
 import androidx.media3.common.MediaItem
 import androidx.media3.common.Player
 import androidx.media3.common.util.UnstableApi
@@ -67,6 +69,18 @@ class ExoPlayerBuilder(
             .apply {
                 setMediaSourceFactory(CustomMediaSourceFactory(videoCache, dataSourceFactory))
                 setLoadControl(feedTunedLoadControl())
+                // Let ExoPlayer own audio focus: pause when a call or another media app takes over
+                // (which flips isPlaying false, so V4V streaming payments stop with it) and duck for
+                // transient interruptions. USAGE_MEDIA so the system treats it as ordinary playback.
+                setAudioAttributes(
+                    AudioAttributes
+                        .Builder()
+                        .setUsage(C.USAGE_MEDIA)
+                        .setContentType(C.AUDIO_CONTENT_TYPE_MOVIE)
+                        .build(),
+                    // handleAudioFocus =
+                    true,
+                )
             }.build()
             .apply {
                 PcmTapRegistry.registerPlayer(this, sink)