New /admin/system-health page (sidebar entry, settings.view) that lists
emails the queue gave up on (status='failed' or pending+retry>=3) with
retry (re-queue) and dismiss (delete) actions. Backend adds /failures,
/failures/email/:id/retry and DELETE on adminSystemHealth. First source
is email failures (the original trigger — quote_sent template errors
left invoices unsent for 14h with no signal); more sources can be added.