fix: restore real-time event log streaming and fix message content loss

[zzzhangqi]

• Restore message.Content field to ensure log messages have actual content
• Fix subscription timing: register channel before pushing history to prevent real-time message loss
• Add rebuildMessageContent to reconstruct Content for file-read messages
• Increase channel buffer from 10 to 1024 to reduce blocking
• Apply same fixes to both readEventBarrel and dockerLogEventBarrel

This fixes the issue where build logs were not streaming in real-time, caused by commit 0b96e9b which removed Content field to save memory. The new approach maintains streaming file persistence while ensuring messages can be properly delivered to WebSocket clients.