## Description
The conversation sidebar counts (`GET /conversations/meta`) are polled
on a debounced cadence selected from the last known conversation count
(`allCount` in the `conversationStats` store).
`allCount` starts at `0` and is only set after a **successful** meta
response. When the meta query is slow or failing under load, `allCount`
stays `0`, which selected the fastest cadence, so every client polled as
aggressively as possible exactly when the endpoint was already
struggling. On large accounts with many concurrent agents this
multiplies into sustained load that keeps the query slow, a
self-reinforcing loop.
This PR makes two changes:
1. **Treat an unknown count (`0`) as a large account** and poll at the
slowest cadence instead of the fastest (`getMetaDebounceKey`, extracted
as a pure, testable function). This breaks the failure loop.
2. **Raise the debounce intervals across all tiers** so counts are
polled less frequently under sustained activity:
- fast: max 1 poll / 2s -> 1 / 5s
- mid: 1 / 10s -> 1 / 20s
- slow (large/unknown accounts): 1 / 20s -> 1 / 30s
First-load counts still render immediately (the debounce fires on the
leading edge for the very first call), so the higher intervals only
affect the sustained poll rate, not initial render.
## Type of change
- [x] Bug fix (non-breaking change which fixes an issue)
