Live widget updates via SSE without full page reload

Description

What problem would this feature solve?

The dashboard only refreshes widget data when the browser loads or reloads the page. If you leave it open, it goes stale - monitor statuses, prices, feed items, etc. stay frozen until you press F5. For a dashboard meant to be glanced at passively (on a wall display, a second monitor, a phone), this defeats the purpose.

Are there any potential downsides?

• Each open browser tab holds a persistent HTTP connection to the server. On most self-hosted setups this is negligible, but it's worth knowing.

• Widgets that render relative timestamps (e.g. "3m ago") will trigger a DOM update every time they're re-fetched even if the underlying data hasn't changed, because the rendered HTML differs. Minor visual flicker, no functional impact.

• Reverse proxies (nginx, Caddy, Traefik) need to be configured to allow long-lived connections and disable response buffering on the /api/events path, otherwise SSE silently breaks.

  What would the configuration look like?

server: live-updates: true # default: false

One flag. Off by default - disabling it leaves behaviour 100% identical to today, with no new goroutines or endpoints.

Are there any existing examples in other software?

• Grafana refreshes panels on a configurable interval via polling
• Home Assistant uses WebSockets to push state changes to the dashboard in real time
• Netdata streams metrics to the browser via SSE
• SSE is the simpler, HTTP-native alternative to WebSockets for this use case - one persistent connection, server pushes, no special protocol

External documentation relevant to implementation:

• MDN: Server-Sent Events
• MDN: EventSource API
• nginx: proxy_read_timeout and X-Accel-Buffering: no header to disable buffering for SSE connections

Anything else relevant?

There is a previous PR (#955) that attempted something similar. Its main issues were: a global event hub that broke hot-reload, auth checked after ID lookup (enabling widget ID enumeration), widgets being polled on a fixed interval ignoring their configured cache duration, and event replay sent to newly connected clients causing spurious refetches on page load. A correct implementation should address all of these.