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

controller/midjourney.go

@@ -130,6 +130,7 @@ func UpdateMidjourneyTaskBulk() {
 				if !checkMjTaskNeedUpdate(task, responseItem) {
 					continue
 				}
+				preStatus := task.Status
 				task.Code = 1
 				task.Progress = responseItem.Progress
 				task.PromptEn = responseItem.PromptEn
@@ -172,18 +173,16 @@ func UpdateMidjourneyTaskBulk() {
 						shouldReturnQuota = true
 					}
 				}
-				err = task.Update()
+				won, err := task.UpdateWithStatus(preStatus)
 				if err != nil {
 					logger.LogError(ctx, "UpdateMidjourneyTask task error: "+err.Error())
-				} else {
-					if shouldReturnQuota {
-						err = model.IncreaseUserQuota(task.UserId, task.Quota, false)
-						if err != nil {
-							logger.LogError(ctx, "fail to increase user quota: "+err.Error())
-						}
-						logContent := fmt.Sprintf("构图失败 %s，补偿 %s", task.MjId, logger.LogQuota(task.Quota))
-						model.RecordLog(task.UserId, model.LogTypeSystem, logContent)
+				} else if won && shouldReturnQuota {
+					err = model.IncreaseUserQuota(task.UserId, task.Quota, false)
+					if err != nil {
+						logger.LogError(ctx, "fail to increase user quota: "+err.Error())
 					}
+					logContent := fmt.Sprintf("构图失败 %s，补偿 %s", task.MjId, logger.LogQuota(task.Quota))
+					model.RecordLog(task.UserId, model.LogTypeSystem, logContent)
 				}
 			}
 		}