feat(admin): ask the panel to restart, from verdande's own settings

A container cannot replace its own image from the inside, so "update" is really
"ask whoever owns the container to recreate it" — Yggdrasil pulls `:latest` on the
way, which is what makes a restart an update at all. This removes the second
browser tab, not the panel's authority: the panel still does the work and still
records who asked.

Three settings, needed together, and the page names whichever are missing — an
operator reading "not configured" should not have to go and find out which of
three. The token is never sent to a browser; the page is told only whether it is
set.

Answers 202 rather than waiting. The panel stops this container as part of
answering, so the reply may never arrive, and a request cut off mid-flight is the
successful case wearing a failure's clothes. The interface believes neither answer
and polls /healthz until it responds.

Administrators only, and sessions only. A leaked API token that could restart the
server is a denial of service in one request.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/configuration.md

@@ -60,6 +60,18 @@ own mailbox from **Settings → Gmail**. The redirect URI is derived from
 | Variable | Default | What it does |
 |---|---|---|
 | `VERDANDE_UPDATE_CHECK` | `false` | Ask GitHub whether a newer release exists. |
+| `VERDANDE_PANEL_URL` | — | The Yggdrasil panel this instance runs under. |
+| `VERDANDE_PANEL_TOKEN` | — | An API token from that panel, belonging to somebody with control of this server. |
+| `VERDANDE_PANEL_SERVER_ID` | — | Which server to restart — the id in the panel's URL. |
+
+All three together let **Indstillinger → Notifikationer** restart this instance,
+which is also how it picks up a new version: a container cannot replace its own
+image, so the panel recreates it and pulls `:latest` on the way. Any one of the
+three alone does nothing, and the page names the ones that are missing.
+
+The token is a panel credential with control over that server. Anybody who can
+read this environment can restart it — which is true of anybody with the host
+anyway — and it is never sent to a browser.
 
 Off unless you turn it on. When on, verdande asks the public GitHub releases API
 at most every six hours and sends nothing about your instance — no identifier, no