v0.6.9: ask the use case, smart topology for game assets

• The server instructions ask what the model is for before picking models when the user hasn't said, and send a game asset or a face-count budget to smart topology (meshy-t2.5 / meshy-t2 with target_polycount), also on meshy_text_to_3d, instead of a meshy-7.1 model remeshed down (#27).
• meshy_text_to_3d's description carries the same steer and the Models page link.