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

model/midjourney.go

@@ -157,6 +157,17 @@ func (midjourney *Midjourney) Update() error {
 	return err
 }
 
+// UpdateWithStatus performs a conditional UPDATE guarded by fromStatus (CAS).
+// Returns (true, nil) if this caller won the update, (false, nil) if
+// another process already moved the task out of fromStatus.
+func (midjourney *Midjourney) UpdateWithStatus(fromStatus string) (bool, error) {
+	result := DB.Where("status = ?", fromStatus).Save(midjourney)
+	if result.Error != nil {
+		return false, result.Error
+	}
+	return result.RowsAffected > 0, nil
+}
+
 func MjBulkUpdate(mjIds []string, params map[string]any) error {
 	return DB.Model(&Midjourney{}).
 		Where("mj_id in (?)", mjIds).