nebula-mesh: API endpoints lack ownership checks, enabling cross-operator privilege escalation

nebula-mesh is a self-hosted control plane for Slack Nebula mesh virtual private network. Prior to version 0.3.4, the `/api/v1/*` route surface trusts the bearer token alone for authorisation on most endpoints. The codebase itself admits this at `internal/api/hosts.go:384`: "API trusts the bearer token for authorisation; per-CA ownership is enforced only in the Web layer." The Web UI gates state-changing routes through `loadAccessibleCA` (`internal/web/cas.go`); CA-management endpoints in `internal/api/cas.go` ALSO have proper `canAccessCA` gates. The gap is on the host, network, firewall, mobile-bundle, and most operator endpoints. Combined with the per-operator CA model from ADR 0002, this gives any non-admin operator API key broad cross-tenant access — instant privilege escalation in the worst case. Version 0.3.4 fixes the issue.