fix(pipeline): premature state update + reject 재생성 알림

버그1: /feedback approve가 bg task 시작 전에 state를 next_pending으로 set →
polling이 빈 video_url로 알림 발송. bg task의 run_step이 state를 set하도록
일임 — 이중 update 제거.

버그2: reject 후 같은 *_pending 상태로 재생성됐을 때 dedupe에 막혀 알림이
안 감. dedupe 키에 feedback_count_per_step[step]을 포함 — 재생성마다
count가 증가하므로 키가 달라져 재알림 동작.

music-lab/app/main.py

@@ -1009,11 +1009,19 @@ async def feedback(pid: int, req: FeedbackRequest, bg: BackgroundTasks):
 
     if req.intent == "approve":
         from .pipeline.state_machine import next_state_on_approve
-        next_st = next_state_on_approve(state)
-        _db_module.update_pipeline_state(pid, next_st)
+        # Validate transition is legal
+        try:
+            next_st = next_state_on_approve(state)
+        except ValueError as e:
+            raise HTTPException(400, str(e))
         next_step = _state_to_step(next_st)
         if next_step:
+            # bg task will set state to the new *_pending when step completes
             bg.add_task(orchestrator.run_step, pid, next_step)
+        else:
+            # No step to run — fall through to direct state update
+            # (defensive — current code paths don't hit this)
+            _db_module.update_pipeline_state(pid, next_st)
         return {"ok": True}
 
     elif req.intent == "reject":