Allow a rolling Call for Speakers not tied to a single dated event

### Use case / problem

Recurring community groups often run a **continuous** Call for Speakers and only fix a meetup's date **after** a speaker is confirmed, the event date is aligned with the speaker's availability rather than set in advance.

Today a Call for Speakers can only be opened on a **concrete event**, which assumes a fixed date (and RSVP/location). For a rolling call this forces organizers into one of two awkward workarounds:

1. **Create a placeholder event with a fake/far-future date** purely to host the CFS, then edit the date later. The public page then shows a date that isn't real, which is confusing for people submitting talks.
2. **Open a new CFS on every event** and re-solicit or manually move proposals each time, so there's no single place to manage the proposal pool.
### Current behavior

A CFS is attached to an event, and events assume a concrete date. There's no way to collect proposals independently of a specific dated event.

### Proposed solution

Support a **group-level / "rolling" Call for Speakers** that is not bound to a single dated event:

- Organizers open **one standing CFS** for the group.
- Submitted proposals accumulate in a **single pool**, reviewable at any time (no fixed deadline).
- Accepted talks can then be **assigned/scheduled to an event** whose date is arranged with the speaker.
- The public CFS page can communicate that there is **no fixed date** and that event dates are aligned with speakers.
This matches the real workflow of recurring meetups and removes the need for placeholder events with unreal dates.

### Alternatives considered

- **Placeholder container event** with a far-future date and a description explaining it's a CFS container (current workaround works, but requires a fake date and is confusing to submitters).
- **External CFS tooling** (e.g., a dedicated meetups/rolling-call mode in another platform) linked from OCG works, but moves the proposal pool out of OCG and splits the organizer workflow.
### Related

- #578 seems to share the same underlying assumption that events are concrete, dated, RSVP-able objects. A rolling CFS is another case where that assumption doesn't fit.
### Additional context

Concrete example: a local CNCF group that aligns each meetup's date with the confirmed speaker. A standing, group-level CFS would fit this far better than a per-event one, and would avoid publishing placeholder dates that attendees might mistake for a real event.