perf: set watermarks even when no archives found to prevent re-scanning

Previously, channels/topics with no new archives never had their
watermark updated. This meant every cycle re-scanned all messages from
scratch just to discover nothing new — especially costly for the 1079-
topic Model Printing Emporium forum.

- Add maxScannedMessageId to ChannelScanResult (highest msg ID seen)
- Set channel watermark to scan boundary when no archives are found
- Set topic watermark to scan boundary when no archives are found
- Fall back to scan watermark when archive processing doesn't advance it

After one full cycle, subsequent cycles will skip already-scanned
messages via the early-exit boundary check, dramatically reducing
TDLib API calls on channels with mostly non-archive content.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

worker/src/tdlib/topics.ts

@@ -178,6 +178,7 @@ export async function getTopicMessages(
   const archives: TelegramMessage[] = [];
   const photos: TelegramPhoto[] = [];
   const boundary = lastProcessedMessageId ? Number(lastProcessedMessageId) : null;
+  let maxScannedMessageId: bigint | null = null;
 
   let currentFromId = 0;
   let totalScanned = 0;
@@ -239,6 +240,12 @@ export async function getTopicMessages(
 
     totalScanned += result.messages.length;
 
+    // Track highest message ID (first message = newest, since results are newest-first)
+    const batchMaxId = BigInt(result.messages[0].id);
+    if (maxScannedMessageId === null || batchMaxId > maxScannedMessageId) {
+      maxScannedMessageId = batchMaxId;
+    }
+
     for (const msg of result.messages) {
       // Check for archive documents
       const doc = msg.content?.document;
@@ -302,6 +309,7 @@ export async function getTopicMessages(
     archives: archives.reverse(),
     photos: photos.reverse(),
     totalScanned,
+    maxScannedMessageId,
   };
 }