Product Tours - priority assignment

Feature request

Is your feature request related to a problem?

users may have many product tours running at the same time, or may have long-running tours/announcements they want to replace with more urgent messages.

Describe the solution you'd like

bare minimum: provide deterministic ordering for which product tour will take precedence when multiple are eligible

better: allow users to assign a priority to each product tour / announcement, and when there are eligibility collisions, break ties with priority

related, larger scope: https://github.com/PostHog/posthog/issues/47965

Describe alternatives you've considered

Additional context

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 by reviewing related issue #47965 and locating the existing product-tour eligibility flow. Define deterministic collision ordering first; if priorities are implemented, done means eligible tours consistently select the highest-priority announcement. No files or tests are named, so identify the relevant implementation and test coverage before estimating the change.