[chore] check-backend-ci-cache.sh 補強 artifact pipeline 覆蓋範圍

[nurockplayer]

背景

PR #364 把 backend CI 改為 artifact pipeline（docker save → upload-artifact → download-artifact → docker load），但 scripts/check-backend-ci-cache.sh 只驗證舊 Buildx/cache 關鍵字，未涵蓋新步驟。日後若誤刪 artifact pipeline 相關步驟，guard 不會抓到 regression。

Source of truth：PR #364 cross-review (Major)

任務

在 scripts/check-backend-ci-cache.sh 補上以下 require_in 檢查：

• actions/upload-artifact — backend-build 必須上傳 artifact
• actions/download-artifact — backend job 必須下載 artifact
• docker save — export 步驟存在
• docker load — load 步驟存在
• backend-image — artifact name 一致

同時確認現有的 Buildx/cache 關鍵字檢查仍適用（這些都在 backend-build job，不在 backend job）。

完成條件

• bash scripts/check-backend-ci-cache.sh 在目前 ci.yml 通過
• 若手動移除任一 artifact 步驟，script 會回傳 non-zero

本票明確不做

• 不修改 CI workflow 本身
• 不修改其他 script

[YUAN-117]

ci.yml 目前尚無 artifact pipeline（docker save/upload-artifact 等步驟），需先實作 artifact pipeline 後此票才有意義，目前列為 backlog。

[nurockplayer]

Closing as superseded by PR #438.

#385 originally asked to add guard checks for the backend image artifact pipeline (docker save / actions/upload-artifact / actions/download-artifact / docker load / backend-image). PR #438 intentionally removed that artifact roundtrip and switched backend unit tests to native Go, so those positive artifact-pipeline checks are no longer the desired direction.

The current guard script now rejects the removed artifact roundtrip markers (actions/download-artifact, docker load, backend-image) instead. If we want to strengthen CI guard behavior further, #284 is the more accurate follow-up for cache guard gating.