Top supervisor for the barrel server.

Children (one_for_one):

1. barrel_server_mcp_live - the live-query bridge (owns every MCP-created subscription).
2. barrel_server_mcp - the MCP tool registrar (before the HTTP service so /mcp never serves an empty registry).
3. barrel_server_http - the livery HTTP service.

Open database handles are owned by Barrel's database lifecycle manager (barrel_dbs, in the barrel application); handlers reach it through the barrel_server_dbs shim.