fix: address code review issues for embeds implementation

- Fix ?? to || in metadataFetcher.ts to handle empty strings from Cheerio
- Fix stale embeds on message edit: delete old embeds synchronously before
  broadcast, then resolve new ones async (all 4 edit paths: REST+WS, msg+DM)
- Revert unrelated MessageList.tsx scroll threshold change (5000 not 150)
- Remove duplicate embed indexes from migrateAddIndexes (kept standalone ones)

packages/web/src/components/chat/MessageList.tsx

@@ -134,7 +134,7 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
           if (el) {
             el.scrollIntoView({ block: 'start' });
             const dist = container.scrollHeight - container.scrollTop - container.clientHeight;
-            const near = dist < 150;
+            const near = dist < 5000;
             setIsNearBottom(near);
             isNearBottomRef.current = near;
             return;
@@ -218,7 +218,7 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
 
     // Check if near bottom
     const distanceFromBottom = container.scrollHeight - container.scrollTop - container.clientHeight;
-    const nearBottom = distanceFromBottom < 150;
+    const nearBottom = distanceFromBottom < 5000;
     setIsNearBottom(nearBottom);
     isNearBottomRef.current = nearBottom;