backstage release v3.9.1

(agent-platform) Answer 404, not 502, for avatars of an installation that serves none by @teemow — For operators: on an installation that serves no agent avatars, the portal’s avatar requests answer 404, as for an agent without one, instead of a 502 and a logged bad gateway per image on every page listing agents.
(app) Catalog background reads never open Login Required by @teemow — After the portal sign-in expired, pages such as the catalog and Settings no longer open the Login Required dialog by themselves from background reads. It opens only when you do something, like a click or a key press.
(app) Treat notification paths starting // or /\ as external by @teemow — A notification link whose path starts with // or /\ no longer sends you to another website from the notification bell; it stays on the portal.
(ui-react) Render Mermaid blocks in markdown as diagrams by @teemow — Mermaid code blocks in markdown, such as in Hive’s Knowledge documents, plans, pull request descriptions and READMEs, show as diagrams instead of their source. An invalid diagram shows its source with Mermaid’s error.

(app) Wait for the tab’s first read in the upstream inbox tests by @teemow — Fixes intermittent failures of the notification inbox tests in CI, which had blocked a release candidate’s publishing.
(chart) Size the database pools so a rollout fits the CNPG max_connections by @teemow — For operators: with database.engine: postgresql, a rolling update no longer runs out of database connections. The chart sets fixed plugin pools and max_connections 400 on the CNPG cluster, and fails to render a replica count that would not fit. The upgrade restarts the CNPG instances once.
(e2e) Session-expiry spec uses dismissInboxToasts by @teemow — Fixes the type check that failed on main after two merges crossed, so releases publish again.

Full Changelog: https://github.com/giantswarm/backstage/compare/v3.9.0...v3.9.1