feat(setup-checks): let ExApps surface setup checks in the admin overview

[oleksandr-nc]

Implements #888.

ExApps can now surface admin-facing setup checks in Settings > Administration > Overview, next to the
core ones (e.g. an ExApp warning that it isn't configured correctly). An ExApp opts in with
POST /api/v1/setup_check (and DELETE to opt out); app_api registers a single aggregate "External
Apps" ISetupCheck, and the results come from each opted-in ExApp's /setup_checks endpoint. A down
or slow ExApp shows "not responding".

The SetupCheck page itself does no HTTP. The fan-out to ExApps runs entirely in the background (a
10-min TimedJob plus a one-off job that an admin's page visit enqueues, stale-while-revalidate) and
stores the results in IAppConfig; the page just reads those, filtered to the currently opted-in and
enabled ExApps. So a slow, down, or errored ExApp can never slow down the admin page.

Notes:

• No new table or migration: the opt-in marker and the computed results both live in IAppConfig.
• ExApp text is escaped and link URLs validated before rendering; response size, string length, and entry count are bounded.
• Deploy/init errors stay out of scope (already shown on the ExApp management page).

Draft: the nc_py_api helper (a @setup_check decorator) and docs are a follow-up; this is the
server/app_api side. NC35-only, not backported.