feat: Add play next functionality to player service and API

app/api/v1/player.py

@@ -249,3 +249,21 @@ async def get_state(
             status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
             detail="Failed to get player state",
         ) from e
+
+
+@router.post("/play-next/{sound_id}")
+async def add_to_play_next(
+    sound_id: int,
+    current_user: Annotated[User, Depends(get_current_active_user_flexible)],  # noqa: ARG001
+) -> MessageResponse:
+    """Add a sound to the play next queue."""
+    try:
+        player = get_player_service()
+        await player.add_to_play_next(sound_id)
+        return MessageResponse(message=f"Added sound {sound_id} to play next queue")
+    except Exception as e:
+        logger.exception("Error adding sound to play next queue: %s", sound_id)
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail="Failed to add sound to play next queue",
+        ) from e