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>

worker/src/tdlib/download.ts

@@ -68,6 +68,8 @@ export interface ChannelScanResult {
   photos: TelegramPhoto[];
 }
 
+export type ScanProgressCallback = (messagesScanned: number) => void;
+
 /**
  * Fetch messages from a channel, stopping once we've scanned past the
  * last-processed boundary (with one page of lookback for multipart safety).
@@ -82,13 +84,15 @@ export async function getChannelMessages(
   client: Client,
   chatId: bigint,
   lastProcessedMessageId?: bigint | null,
-  limit = 100
+  limit = 100,
+  onProgress?: ScanProgressCallback
 ): Promise<ChannelScanResult> {
   const archives: TelegramMessage[] = [];
   const photos: TelegramPhoto[] = [];
   const boundary = lastProcessedMessageId ? Number(lastProcessedMessageId) : null;
 
   let currentFromId = 0;
+  let totalScanned = 0;
 
   // eslint-disable-next-line no-constant-condition
   while (true) {
@@ -103,6 +107,8 @@ export async function getChannelMessages(
 
     if (!result.messages || result.messages.length === 0) break;
 
+    totalScanned += result.messages.length;
+
     for (const msg of result.messages) {
       // Check for archive documents
       const doc = msg.content?.document;
@@ -132,6 +138,9 @@ export async function getChannelMessages(
       }
     }
 
+    // Report scanning progress after each page
+    onProgress?.(totalScanned);
+
     currentFromId = result.messages[result.messages.length - 1].id;
 
     // Stop scanning once we've gone past the boundary (this page is the lookback)
@@ -144,7 +153,7 @@ export async function getChannelMessages(
   }
 
   log.info(
-    { chatId: chatId.toString(), archives: archives.length, photos: photos.length },
+    { chatId: chatId.toString(), archives: archives.length, photos: photos.length, totalScanned },
     "Channel scan complete"
   );