Implement UploaderBacklogCleared soak criterion

## Summary

The `UploaderBacklogCleared` soak criterion in `internal/daemon/consensus/criteria.go` is currently a stub that always returns `(false, nil)`. It needs to query the record stream uploader for its backlog status to determine whether all record stream events from the old CN have been uploaded to mirror nodes.

## Part of epic

Part of epic #499 — solo-provisioner-daemon Core

| SP | 3 |
|---|---|

## Acceptance criteria

- `UploaderBacklogCleared.Check()` queries the record stream uploader (via an appropriate API or health endpoint TBD by the uploader team) for backlog status
- Returns `(true, nil)` when no backlog events remain from the old CN
- Returns `(false, nil)` when backlog events are still pending (not an error)
- Returns `(false, err)` only on I/O / connectivity errors
- Unit-tested with a fake uploader client

## Context

This criterion is part of the soak-watcher goroutine implemented in story #520. All soak criteria must be green before the daemon triggers decommission of the old CN.

The record stream uploader endpoint/API to query needs to be confirmed with the uploader team.