ephemeral collections in replay

Feature request

there are a number of places where it would be cool to have an easily embeddable / displayable collection of recordings from a known list of session ids

currently we have

• saved filters -> the user knows conditions they want to come back to, but we don't know what recordings will show
• collections -> fixed set of sessions the user added to a collection
• ephemeral collection -> a known set of session ids that we want to put in place that we generate (not the user)

they're ephemeral to the user since we wouldn't show them in the collections page, so they can only get to them by repeating the flow that got them to the list

example use-case

• user viewing history to show which recordings they've watched
• insight datapoints instead of / alongside the actor modal, jump from a funnel drop-off to a list of recordings to watch
• in notebooks when showing user painpoints in a session summary

since these may be displayed outside of the recording context we probably also want a "mini playlist" experience

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]

The issue names no files, tests, or entry points; begin by locating the existing saved-filter and collection flows, then trace how recording session IDs are represented and displayed. Clarify the ephemeral collection lifecycle, the external entry points, and the scope of the proposed mini-playlist before implementation; done should cover the agreed use cases without exposing these collections on the collections page.