Add Phase 3 multi-user howto set (v1.0) - Howto-Migrate-to-multi-user.md: backup, dry-run, upgrade, rollback, migration 019 troubleshooting - Howto-Add-a-second-user.md: local/OIDC/proxy users, roles, promote/demote, delete safely - Howto-Recover-admin-access.md: API recovery, direct DB update (Docker + K8s), password reset, API key retrieval - Howto-CSRF-tokens.md: session-cookie preflight, migration from X-Requested-With, API-key exemption Update Home.md and _Sidebar.md with "Multi-user — v1.0.0" section.