Reduces database pressure from filtered unread-count cache rebuilds
during high-traffic account rollouts by serving stale snapshots longer
and limiting inline saved-filter rebuild fanout.
## Closes
None
## What changed
- Increase filtered unread-count refresh throttling from 30 seconds to 5
minutes.
- Increase the stale snapshot window from 30 minutes to 1 hour.
- Reduce inline saved-filter count rebuilds per request from 10 to 3.
- Update unread-count specs to assert refresh and stale behavior through
the shared constants.
## How to test
- Enable `conversation_unread_counts` and `unread_count_for_filters` for
an account with conversation custom filters.
- Open the dashboard and verify unread-count badges still return values.
- Mutate conversations and verify stale filtered counts are served while
rebuilds are throttled, instead of repeatedly rebuilding every 30
seconds.
