refactor(task): add CAS-guarded updates to prevent concurrent billing conflicts

Replace all bare task.Update() (DB.Save) calls with UpdateWithStatus(),
which adds a WHERE status = ? guard to prevent concurrent processes from
overwriting each other's state transitions.

Key changes:

model/task.go:
- Add taskSnapshot struct with Equal() method for change detection
- Add Snapshot() method to capture pre-update state
- Add UpdateWithStatus(fromStatus) using DB.Where().Save() for CAS
  semantics with full-struct save (no explicit field listing needed)

model/midjourney.go:
- Add UpdateWithStatus(fromStatus string) with same CAS pattern

service/task_polling.go (updateVideoSingleTask):
- Snapshot before processing upstream response; skip DB write if unchanged
- Terminal transitions (SUCCESS/FAILURE) use UpdateWithStatus CAS:
  billing/refund only executes if this process wins the transition
- Non-terminal updates also use UpdateWithStatus to prevent overwriting
  a concurrent terminal transition back to IN_PROGRESS
- Defer settleTaskBillingOnComplete to after CAS check (shouldSettle flag)

relay/relay_task.go (tryRealtimeFetch):
- Add snapshot + change detection; use UpdateWithStatus for CAS safety

controller/midjourney.go (UpdateMidjourneyTaskBulk):
- Capture preStatus before mutations; use UpdateWithStatus CAS
- Gate refund (IncreaseUserQuota) on CAS success (won && shouldReturnQuota)

This prevents the multi-instance race condition where:
1. Instance A reads task (IN_PROGRESS), fetches upstream (still IN_PROGRESS)
2. Instance B reads same task, fetches upstream (now SUCCESS), writes SUCCESS
3. Instance A's bare Save() overwrites SUCCESS back to IN_PROGRESS

relay/relay_task.go

@@ -444,6 +444,8 @@ func tryRealtimeFetch(task *model.Task, isOpenAIVideoAPI bool) []byte {
 		return nil
 	}
 
+	snap := task.Snapshot()
+
 	// 将上游最新状态更新到 task
 	if ti.Status != "" {
 		task.Status = model.TaskStatus(ti.Status)
@@ -459,7 +461,10 @@ func tryRealtimeFetch(task *model.Task, isOpenAIVideoAPI bool) []byte {
 		// No URL from adaptor — construct proxy URL using public task ID
 		task.PrivateData.ResultURL = taskcommon.BuildProxyURL(task.TaskID)
 	}
-	_ = task.Update()
+
+	if !snap.Equal(task.Snapshot()) {
+		_, _ = task.UpdateWithStatus(snap.Status)
+	}
 
 	// OpenAI Video API 由调用者的 ConvertToOpenAIVideo 分支处理
 	if isOpenAIVideoAPI {