feat: make it possible to see which version is actually running

"I have updated and restarted, but I cannot tell whether it worked" is
not a question the panel could answer. The version was on the startup
line and in the settings page, and nowhere you can reach from outside.

/healthz carries it now. That is the one endpoint answering without a
session, which makes it the only way to ask the question from a terminal
or from the panel itself. The "listening" line repeats it too, because
that is the line the Rune's done_regex matches and therefore the one
sitting at the bottom of the console after a deploy.

The update notice could also never have worked. It reads GitHub's
releases/latest, and this repository had only tags — a tag is not a
release, and that endpoint answers 404 for a repository without any. The
release workflow creates one now, after the image is pushed so a release
never announces a version that failed to build.

VERDANDE_UPDATE_CHECK is a Rune variable, so it can be switched on
without editing the container. Still off by default: it is an outbound
request, and that is a choice to make rather than a default to inherit.

Rune version 1 -> 2, so the panel offers the update.

Note that the check still cannot see anything while the repository is
private — releases/latest is 404 anonymously, exactly as the GHCR package
was. Making the repo public, or teaching the check to carry a token, is a
separate decision.

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

docs/openapi.yaml

@@ -1718,7 +1718,15 @@ paths:
         broken volume reports unhealthy instead of pretending to work.
       security: []
       responses:
-        "200": { description: Healthy. }
+        "200":
+          description: Healthy. Carries the running version — the one thing you can ask without a session.
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  status: { type: string, const: ok }
+                  version: { type: string, example: v0.3.1 }
         "503": { description: Unhealthy. }
 
   /ics/{token}: