feat: web terminal + Headlamp-style K8s browser

[fropa]

Summary

Self-hosted browser terminal (PTY to nodes and pods) plus a Headlamp-style read-only K8s resource browser. Fully self-hosted — no third-party SaaS. Also folds in correctness/cost fixes from the earlier review pass.

Web terminal

• Backend: byte relay over Redis pub/sub (worker-safe), client + agent WS endpoints, TerminalSession audit row, first-message API-key auth (key kept off the URL).
• Agent: hand-rolled stdlib RFC6455 WS client (keeps agent zero-dep) + pty.fork, driven off the existing exec poll.
• Frontend: xterm.js TerminalPanel, Terminal tab on nodes.
• Security (design borrowed from DirectGate, adapted to a trusted self-hosted relay): tenant/node scoped, concurrency cap, idle + max-duration timeouts, fully audited. Shell runs as the agent's restricted user.

K8s browser (read-only)

• Agent ships extended resource kinds (services, configmaps, statefulsets, daemonsets, jobs, ingresses, endpoints, networkpolicies, pvcs, pvs, events, …). Secret values stripped on the host.
• Frontend Resources tab + YAML drawer (js-yaml); pod rows get Shell + YAML actions.
• Shell into pods via kubectl exec, reusing the terminal infra. K8s names regex-validated against argv injection.

Review fixes

• Traces background task now opens its own DB session (was reusing the closed request session).
• datetime.utcnow() → datetime.now(timezone.utc) across routes.
• Claude model tiering: RCA → Opus, chat/topology → Haiku, balanced default.
• Added missing vite-env.d.ts; fixed a pre-existing Dashboard type error → tsc clean.

Status / caveats

• Backend py_compile, agent py_compile, and frontend tsc --noEmit all pass.
• Not yet run live — verify on a real host/cluster: kubectl-exec PTY, systemd hardening vs pty.fork, and large-cluster Redis payload size.
• Deferred (north-star): cross-environment session/network flow (on-prem ↔ pod).

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