audio: raise backingVolume default 0.7 -> 0.8

Bring the backing track up ~1.2 dB so the player tone (leveled to -15.5 LUFS
by RBFinalLeveler) sits with the music instead of dominating it. Part of the
tone-vs-backing balance pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/audio/AudioEngine.h

@@ -406,7 +406,7 @@ private:
 
     // Master output (post-mix) — engine-global, not per-source.
     std::atomic<float> outputGain{1.0f};
-    std::atomic<float> backingVolume{0.7f};
+    std::atomic<float> backingVolume{0.8f};
     // Per-song loudness normalizer for the backing track (applied in
     // renderBackingBlockLocked, pre-fader). Owned + driven by the audio thread.
     BackingLeveler backingLeveler;