Feature request: add manual audience access control for sessions

## Problem

AudienceMeter currently appears to make a session available to the audience based on the scheduled session time and duration. That default is useful, but it can create a poor live-event experience when the time is configured incorrectly, the speaker is in a different time zone, or the event schedule shifts.

In that situation, the speaker may already be in front of the audience with the shared QR code/link, but the session is still unavailable. The only recovery path is to log in, edit timing, or troubleshoot live, which is stressful and visible to the audience.

## Requested feature

Add a manual audience access control for a session, so the speaker can quickly override schedule-based availability.

Suggested model:

- `Auto`: default behavior; audience access follows the scheduled date/time and duration.
- `Open now`: manually makes the session available immediately, regardless of the scheduled time.
- `Closed`: manually closes audience access, regardless of scheduled time.

This could replace or clarify the existing public switch, rather than adding a second, separate control.

## Why this matters

This is mainly an event-day recovery feature. The schedule should still exist and remain the default, but speakers need a reliable one-click fallback when the timing is wrong or the live event changes.

I have already run into this during a real event, where the session was set up but did not become available when expected. A manual open/close control would have prevented the bad audience experience.

## Expected behavior

- Existing sessions continue using schedule-based availability by default.
- Speakers can manually open an upcoming or ended session.
- Speakers can manually close a live session.
- Returning to `Auto` restores normal scheduled behavior.
- Audience join and feedback routes respect the same computed availability.

Since the backend and web app are not open source, I unfortunately can't contribute. But let me know if I can collaborate with you on this.