fix: throttle filtered unread count rebuilds (#14980)

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.

app/services/conversations/unread_counts.rb

@@ -2,9 +2,9 @@ module Conversations::UnreadCounts
   READY_TTL = 24.hours.to_i
   SET_TTL = 25.hours.to_i
   FILTERED_COUNT_FRESH_TTL = 5.minutes.to_i
-  FILTERED_COUNT_STALE_WINDOW = 30.minutes.to_i
+  FILTERED_COUNT_STALE_WINDOW = 1.hour.to_i
   FILTERED_COUNT_REDIS_TTL = FILTERED_COUNT_FRESH_TTL + FILTERED_COUNT_STALE_WINDOW
   FILTERED_COUNT_VERSION_TTL = SET_TTL
-  FILTERED_COUNT_MIN_REFRESH_INTERVAL = 30.seconds.to_i
-  MAX_INLINE_FILTER_BUILDS = 10
+  FILTERED_COUNT_MIN_REFRESH_INTERVAL = 5.minutes.to_i
+  MAX_INLINE_FILTER_BUILDS = 3
 end