Dashboard misses real-time session updates: WS event-type mismatch

[mondominator]

Symptom

New streams take up to 30 seconds to appear on the dashboard even when OpsDec's backend has already processed the session update. WebSocket is connected, but real-time updates are silently dropped and only the 30s polling loop actually refreshes the UI.

Root cause

Event-type mismatch between the backend broadcast and the frontend listener:

• `backend/src/services/monitor.js:1301` broadcasts:
  ```js
  broadcast({ type: 'activity', data: currentSessions });
  ```
• `frontend/src/pages/Dashboard.jsx:117` listens for:
  ```js
  if (data.type === 'session.update') { ... }
  ```

Those strings don't match, so the message is received over the socket and then ignored. The dashboard still updates because `loadData()` runs on a `setInterval(..., 30000)` — but that's not real-time, and it masks the fact that WS-driven updates don't work at all.

Repro

1. Start a new stream on any server OpsDec is monitoring
2. Watch the OpsDec backend logs — `📺 New session started: ...` appears
3. Dashboard does NOT update
4. Wait up to 30s — dashboard finally refreshes on the next poll

Fix options

Pick one:

• Change the backend to broadcast `type: 'session.update'` so it matches the listener
• Or change the frontend to listen for `type: 'activity'` to match the backend
• Ideally settle on one convention across all broadcasts (look for other `broadcast({...})` callsites too)

Why this matters

Without real-time updates, the Dashboard loses most of its value — especially for audiobook sessions where the state changes (play/pause/seek) happen often and the 30s lag is very visible. This is probably why the dashboard has felt "sluggish" on new-stream detection.