Show total match count on Replays list page

The Replays list page (and its underlying API) has no way to see a total count of replays matching the current search, including combined filters like count_errors:>0 release:X. Users can only page through results with cursor-based pagination and count manually.

• organization_replay_index.py uses a cursor-based ReplayPaginator (Link header next/prev only); the response never includes a total count field.
• The frontend (replayIndexContainer.tsx) only reads the Link header for prev/next state, so there's nothing to surface even if a user wanted it.
• There is also no Dashboards WidgetType for replays (views/dashboards/types.tsx only covers discover/issue/metrics/errors/transactions/spans/logs/tracemetrics/preprod-app-size), so a count() aggregate widget over replays isn't possible either.

This makes it hard to answer questions like "how many error-linked replays happened on release X" without manually paging through the list.

Request: add a total match count (or a count() aggregate path via Discover/Dashboards) for the Replays list/search, similar to how Issues search shows counts.

Requested by kpujji via Junior.

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View Junior Session in Sentry

Start with organization_replay_index.py and its cursor-based ReplayPaginator to understand the current response and pagination headers. Then read replayIndexContainer.tsx and views/dashboards/types.tsx to trace frontend handling and existing widget types. Done means filtered Replays searches expose a total match count through the API and make it available in the list or an agreed Discover/Dashboards path.