Reintroduce WebSockets for real-time run, judgement and viewer updates.

[Jackzzey]

Backend

• Re-enables ws("ws/run", RunExecutor::accept); RunExecutor
  broadcasts a ServerMessage for each relevant EventStream event
  (task/run start/end, score updates, submissions, new
  ViewerUpdateEvent).
• ServerMessage now carries optional state / overview /
  teamOverview diffs, so most updates are delivered as data, not
  just a "something changed" signal:
  • TASK_START / TASK_END embed the full evaluation state +
    overview.
  • COMPETITION_UPDATE embeds the overview.
  • TASK_UPDATED (submissions) embeds only the submitting
    team's overview, so one team's submission doesn't require
    re-sending or re-fetching everyone's data.
• AccessManager.canViewEvaluation enforces the same access
  rules on REGISTER as the REST API.
• Fixes a self-deadlock in the old (dead) cleanup-thread code;
  connection state moved to ConcurrentHashMap.

Frontend

• New WebSocketService with reconnect backoff, ping
  keep-alive, and a reconnected$ signal that forces a one-off
  resync after a drop (to cover messages missed while
  disconnected).
• run-admin-view, run-async-admin-view, run-viewer, and the
  judgement viewers apply the embedded
  state/overview/teamOverview diffs directly when present — no
  REST call needed. They only fall back to an HTTP fetch for the
  few message types that carry no diff (lifecycle events like
  TASK_PREPARE/COMPETITION_END/VIEWER_UPDATE), or as a 30s
  safety-net poll.
• New specs covering the diff-application and fallback paths
  for all of the above.

All frontend tests pass (103/103). Manually verified diffs are
applied live (no fetch) on task start/end, score updates, and
submissions, and that reconnect triggers a full resync.