tos.md was baked into the repo and read once at startup, so changing
the terms required a code change and deploy. It's now a Redis-backed
singleton (models/tos.js), editable from a new "Terms of Service" card
on the admin Dashboard, with the bundled tos.md used only as a
one-time seed for new deployments.
- routes/tos.js: GET (any authenticated user) / PUT (app_sso_admin
only) via /api/tos. Saving can optionally reset every user's
tos_accepted flag so they're asked to re-accept -- off by default,
since a wording fix shouldn't re-prompt everyone.
- routes/index.js: /tos and /onboarding now render the live content
instead of a module-level constant computed once at process start.
