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/server/src/db/migrate.ts

@@ -465,9 +465,6 @@ function migrateAddIndexes(db: Database.Database): void {
     // Categories
     'CREATE INDEX IF NOT EXISTS idx_channel_categories_space_id ON channel_categories(space_id)',
 
-    // Embeds
-    'CREATE INDEX IF NOT EXISTS idx_embeds_message_id ON embeds(message_id)',
-    'CREATE INDEX IF NOT EXISTS idx_embeds_dm_message_id ON embeds(dm_message_id)',
   ];
 
   db.exec(indexes.join(';\n'));