feat: Elixir 1.20-rc4/OTP28, wire Phoenix endpoints, router, LiveViews, auth plug
- flake.nix: Elixir 1.20.0-rc.4 via pkgs.path (reproducible, no hardcoded store paths)
- RequireAuth plug (session-based, redirects to /login)
- SessionController: login form, logout, OIDC callback stub
- DashboardLive, ReplicationLive, BillingLive, SupportLive (skeleton)
- Layouts: dark UI, nav, flash messages
- All compile clean on 1.20-rc4 with zero warnings
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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