Add live message scanning progress, channel/topic counters to worker activity

- Add progress callbacks to getChannelMessages and getTopicMessages that
  fire after each page of messages is fetched
- Worker now shows channel progress (e.g. "[2/5] Channel Name") when
  processing multiple source channels
- Worker now shows topic progress (e.g. "topic 3/12") when scanning forums
- Worker now shows live message scanning count during channel/topic scans
  (e.g. "Scanning Channel — 300 messages scanned")
- UI stats line now always shows messagesScanned count
- messagesScanned counter now increments during the scanning phase, not
  just during archive processing

Co-authored-by: xCyanGrizzly <53275238+xCyanGrizzly@users.noreply.github.com>

src/app/(app)/telegram/_components/worker-status-panel.tsx

@@ -233,6 +233,11 @@ function RunningStatus({
             </span>
           </span>
         )}
+        {run.messagesScanned > 0 && (
+          <span>
+            <span className="text-foreground tabular-nums">{run.messagesScanned}</span> messages
+          </span>
+        )}
         {run.zipsIngested > 0 && (
           <span>
             <span className="text-foreground tabular-nums">{run.zipsIngested}</span> ingested