Join pre/prompt/post into the first MiniMax prompt, not only later shots.

The motion prompt field is what Comfy actually runs. Wrappers are now composed into that string on submit, and shot 1 falls back to it if the separate pre/post fields do not arrive.

Co-authored-by: Cursor <cursoragent@cursor.com>

server/utils/studioQueue.ts

@@ -1179,7 +1179,7 @@ export async function startStudioJob(item: StudioJob) {
   }
 
   void runGeneration(job, {
-    prompt: payload.promptMid || payload.prompt,
+    prompt: payload.prompt,
     image: stillFile,
     width: payload.width,
     height: payload.height,