v1.0.406

Ship DocVault health and invoice alert add-ons

An alert add-on is a check job, the alert it reports, and a dashboard page.
Two now ship with DocVault, seeded disabled like the example jobs:

• docvault-health (every 30m): backup age and errors, scheduled jobs that keep
  failing or stopped running, failed outbound email, and data-drive space.
• overdue-invoices (every 6h): timesheet invoices 7+ and 30+ days past due, and
  billable time from earlier months that never made it onto an invoice.

Dashboards are built from a shared kit in examples/alerts/kit: an add-on's
render.js is inlined into kit.html by assemble.ts, previewed with harness.ts,
and parsed by check.ts. On boot each bundled dashboard is copied to
DATA_DIR/alerts//dashboard.html once; a marker keeps a deleted dashboard
from coming back, and an existing one is never overwritten.

The kit replaces the old single-file starter dashboard. examples/alerts is
left out of lint like scripts/: its render.js files only make sense inlined
into the kit.