Deferred security/correctness items from #48

[LiukScot]

Deferred security/correctness items captured during review of #48. Full patches + decision notes in PR body of #48.

• HIGH — Rate limiting on login endpoint — internal/server/server.go:186, internal/auth/auth.go:64. Per-IP token bucket via golang.org/x/time/rate.
• HIGH — Unbounded SELECT in system_history.go Query() — internal/collectors/system_history.go:212. Move dedup/merge into SQL GROUP BY.
• MEDIUM — Password echoed to terminal in user-cli — scripts/user-cli.go:42. Use golang.org/x/term.ReadPassword.
• MEDIUM — StartBroadcastLoop ticker never stopped — internal/server/ws.go:145. Accept context.Context, defer ticker.Stop().
• LOW — WebSocket read pump has no read deadline — internal/server/ws.go:130. Set read deadline + reset in pong handler.