feat: add L1_PUBLIC_URL config for many-to-many L1/L2 support

- Add L1_PUBLIC_URL environment variable
- UI publish sends L1 URL to L2 so it knows where to fetch run data
- Update .env.example with new config

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

@@ -1,5 +1,8 @@
 # L1 Server Configuration
 
+# This L1 server's public URL (sent to L2 when publishing)
+L1_PUBLIC_URL=https://l1.artdag.rose-ash.com
+
 # L2 server URL (for authentication and publishing)
 L2_SERVER=https://artdag.rose-ash.com