Participant count is incorrect

The participant counter on the top left corner of element call is incorrect.

- The counter uses the call.member events to parse the current non expired events
- The exparation is computer by `unsigned.age` or the `origin_server_ts` as a fallback if no `unsigned.age` is unavailable.

The age is only properly set right after the evnt is fetched from the HS.
We suspect, this collides with the idea of the sync cache from the js-sdk.

It should only store events with the `localTimestamp` and not the `unsigned.age`.

Trace the participant counter from call.member event handling through the js-sdk sync cache, focusing on unsigned.age, origin_server_ts, and localTimestamp. Confirm how expiration is calculated, then update the stored event data so the counter remains correct after cached events are reused; done means expired members are excluded consistently.

Written by the indexing model from the issue text.