Feature: View and unarchive archived LLM prompts

Feature request

Once a prompt is archived, there's no way to view archived prompts in the UI or filter for them via the API. There's also no unarchive option, so archiving is effectively permanent with no recovery path.

Please describe.

1. A way to view all archived prompts (e.g. a toggle or filter in the Prompt management UI, or an archived=true query parameter on the list API endpoint).
2. An unarchive action to restore an archived prompt to active use, including its versions and labels.

Duplicating the archived prompt – but this only works if the prompt is still accessible, and the content may be lost. Recreating it manually is error-prone.

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

Start with the Prompt management UI and the prompt list API endpoint described in the issue, then trace how archived prompts, versions, and labels are represented. Done means archived prompts can be viewed or filtered, and an unarchive action restores a prompt with its versions and labels.