feat: add gallery link maintenance notifications

[poterpan]

Summary

• add anonymous Google Drive gallery link checking with machine-readable reports
• add transition-based Discord notifications for permission and invalid links
• add GitHub Actions Cache state handling and silent PR validation
• document secret setup and maintenance workflow behavior

Validation

• yarn prettier passed
• checker runs successfully and currently reports existing restricted links (expected non-zero)
• notifier smoke test passed
• node --check passed for both scripts
• git diff --check passed
• yarn build passed in the final verification run (with the existing CSS @import warning)

The Discord webhook secret is intentionally not included in this PR. Maintainers must configure DISCORD_GALLERY_WEBHOOK_URL in the upstream repository Actions secrets before enabling maintenance notifications.

[poterpan]

失敗原因是預期中的 gallery checker 結果：

yarn test:gallery-links
exit code 1

目前 repository 中仍有一些 Google Drive folder 回報 Permission denied，而 checker 的設計是只要有任何不可存取連結，就以 exit code 1 結束。因此這代表：

• PR checker workflow 已成功註冊並執行
• GitHub Actions checkout、Node、Yarn 與 checker 都正常運作
• checker 正確偵測到目前的權限問題
• 並非 workflow 設定錯誤

Ready to merge.