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/app/cloud/connector.py

@@ -382,7 +382,26 @@ class CloudConnector:
             async with event_bus.subscribe() as queue:
                 while True:
                     event = await queue.get()
-                    if only is not None and not event_for_panel(event, only):
+                    if only is not None and event.get("type") == "dashboard_changed":
+                        # What this screen shows may have just changed under
+                        # it. Rescope, then resend the snapshot so a dashboard
+                        # it has only now been assigned draws values instead of
+                        # blanks. ``or set()`` is the point: a panel that was
+                        # deleted scopes to nothing, and None would widen this
+                        # socket to everything on the bus.
+                        only = panel_scope(token, self._app) or set()
+                        if controller is not None:
+                            catch_up = snapshot_payload(controller, only)
+                            await socket.send(
+                                _dump(
+                                    {
+                                        "op": "ws_msg",
+                                        "id": stream_id,
+                                        "text": _dump(catch_up),
+                                    }
+                                )
+                            )
+                    elif only is not None and not event_for_panel(event, only):
                         continue
                     await socket.send(
                         _dump({"op": "ws_msg", "id": stream_id, "text": _dump(event)})