Make Terms of Service editable at runtime by admins (closes #39)

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.

nodejs/app.js

@@ -80,6 +80,7 @@ app.use('/api/group', middleware.auth, require('./routes/group'));
 app.use('/api/service-account', middleware.auth, require('./routes/service_account'));
 app.use('/api/notification', middleware.auth, require('./routes/notification'));
 app.use('/api/update-check', middleware.auth, require('./routes/update_check'));
+app.use('/api/tos', middleware.auth, require('./routes/tos'));
 
 // Self-service API tokens (PATs) — owner-scoped, no admin group required.
 app.use('/api/api-token', middleware.auth, require('./routes/api_token'));