Hold a new dashboard back until someone publishes it

A dashboard went live the moment it was created — an empty document straight to
the panels — while a new flow starts as a draft. It now works the way flows do:
published means `dashboard.json` exists, so every dashboard on every running
installation is already published and nothing needs migrating. Only the ones
created from here on start as drafts.

Mirroring FlowStore turned up a latent 500: discarding the draft of a dashboard
that had never been published unlinked its only file, and the read that followed
raised out of a 200 handler. It answers 400 now, the way a flow does.

Publishing all of them was 2N requests, because a publish has to name the
version it expects and the summaries did not carry one. They do now — and so do
the flow summaries, which had the same defect nobody had written down.

A panel had no way to hear about any of this. A publish, or a change to which
dashboards a panel carries, now puts one event on the bus and the screen
refetches what changed: no reload, so a wall display never blanks or asks for
its credential again. The subtle half is that a socket's message allowlist was
computed once at handshake — a reassigned panel would have fetched its new
document and then shown tiles that never updated.

The panels dialog logged non-superusers out. Every write in it needs a
superuser, not only the checkboxes the report mentioned, so the dialog is
read-only for everyone else. The logout itself was `main.tsx` treating 403 as a
dead session, against the contract deps.py spells out: only a 401 ends a
session, and a 403 now says so rather than silently signing someone out.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uq8mtNb97A7praJLyeEYgs

backend/tests/test_cloud.py

@@ -135,6 +135,18 @@ def test_a_panel_scoped_portal_token_reaches_only_its_panel(
             f"{settings.API_V1_STR}/dashboards/{name}", headers=superuser_token_headers
         )
         assert created.status_code in (200, 201, 409), created.text
+        # A new dashboard is a draft, and a panel only reaches what is
+        # published — so promote it before asking as one.
+        version = client.get(
+            f"{settings.API_V1_STR}/dashboards/{name}",
+            headers=superuser_token_headers,
+            params={"draft": "true"},
+        ).json()["version"]
+        client.post(
+            f"{settings.API_V1_STR}/dashboards/{name}/publish",
+            headers=superuser_token_headers,
+            json={"version": version},
+        )
 
     token = portal_token(portal_key, subject="hallway", scope="panel")
     headers = {"Authorization": f"Bearer {token}"}