v0.4.0

Per-device IPMI credentials: each server can carry its own BMC host, username,
password and cipher suite, set from the server's IPMI page or when adding the
server. A blank field falls back to the server-wide default.

- The IPMI page is no longer blocked while IPMI is unreachable (it is the page
  that fixes the credentials), and an empty credential field no longer fails
  with a 422.
- New DELETE /api/servers/{id}/ipmi and the MCP clear_ipmi tool revert a server
  to the defaults; the page can Test connection, Clear override and Retry a
  failed read.