update helper: follow renamed ct/ scripts instead of curling a 404

[MickLesk]

✍️ Description

A container keeps the slug it was built with. When a ct/ script is renamed the slug goes stale, and run_app_update curled a 404 into bash -c "" — did nothing, exited 0, reported success.

Reported for pbs (renamed to proxmox-backup-server). The Alpine merge on 2026-08-18 retired 29 more names, so every container from an alpine-* script before that date is affected too.

Candidates are only accepted when the target script exists, so an unknown slug still fails loudly instead of running another app's updater.

🔗 Related Issue

Refs community-scripts/ProxmoxVE#16989

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – resolver unit-tested against known slugs; not run on a live container.
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.
• API.txt regenerated – n/a, this repo has no API.txt.

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🤖 AI Assistance (X in brackets)

If you used an AI tool (GitHub Copilot, Claude, ChatGPT, etc.) to write or generate any code in this PR, you must confirm compliance below.
Select exactly one option.

• No AI used – Code was written without AI assistance.
• AI was used – The output has been reviewed and corrected, and I take responsibility for it matching this repo's conventions.

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🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing behavior in a way that may require updates in every downstream script.
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, docs/, CONTRIBUTING.md, or other docs.