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/api/routes/test_panels.py

@@ -14,6 +14,22 @@ def _panels(client: TestClient, headers: dict[str, str], config: dict) -> None:
     assert response.status_code == 200, response.text
 
 
+def _dashboard(client: TestClient, headers: dict[str, str], name: str) -> dict:
+    """Create a dashboard and publish it.
+
+    A new one is a draft, and a panel is only ever shown what is published.
+    """
+    created = client.post(f"{DASHBOARDS}/{name}", headers=headers)
+    assert created.status_code == 200, created.text
+    published = client.post(
+        f"{DASHBOARDS}/{name}/publish",
+        headers=headers,
+        json={"version": created.json()["version"]},
+    )
+    assert published.status_code == 200, published.text
+    return published.json()
+
+
 def _pair(client: TestClient, headers: dict[str, str], panel: str) -> dict[str, str]:
     """Walk a device through pairing and return the header it ends up with."""
     started = client.post(f"{PREFIX}/pair").json()
@@ -122,7 +138,7 @@ def test_paired_panel_reaches_only_what_it_shows(
     client: TestClient, superuser_token_headers: dict[str, str]
 ) -> None:
     for name in ("panel_shown", "panel_hidden"):
-        client.post(f"{DASHBOARDS}/{name}", headers=superuser_token_headers)
+        _dashboard(client, superuser_token_headers, name)
     _panels(
         client,
         superuser_token_headers,
@@ -187,7 +203,7 @@ def test_paired_panel_reaches_only_what_it_shows(
 def test_removing_the_panel_revokes_its_credential(
     client: TestClient, superuser_token_headers: dict[str, str]
 ) -> None:
-    client.post(f"{DASHBOARDS}/panel_gone", headers=superuser_token_headers)
+    _dashboard(client, superuser_token_headers, "panel_gone")
     _panels(
         client,
         superuser_token_headers,
@@ -368,10 +384,7 @@ def test_a_panels_socket_carries_only_what_it_draws(
         snapshot_payload,
     )
 
-    client.post(f"{DASHBOARDS}/panel_socket", headers=superuser_token_headers)
-    saved = client.get(
-        f"{DASHBOARDS}/panel_socket", headers=superuser_token_headers
-    ).json()
+    saved = _dashboard(client, superuser_token_headers, "panel_socket")
     saved["pages"] = [
         {
             "id": "main",
@@ -435,6 +448,9 @@ def test_a_panels_socket_carries_only_what_it_draws(
         {"type": "message_value", "name": "house.safe.code"}, only
     )
     assert not event_for_panel({"type": "node_log", "text": "a traceback"}, only)
+    # Except a dashboard being published: that is how a screen hears the
+    # document it draws — or the set of them it was given — has moved.
+    assert event_for_panel({"type": "dashboard_changed", "dashboard": "x"}, only)
 
     # A person's credential is not bounded at all.
     assert panel_scope(superuser_token_headers["Authorization"][7:], client.app) is None