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/views/tos.ejs

@@ -6,6 +6,7 @@
 				<i class="fa-solid fa-file-contract"></i> Terms of Service
 			</div>
 			<div class="card-body">
+				<p class="text-muted small">Last updated: <%= tosUpdatedOnFmt %></p>
 				<%- tosHtml %>
 			</div>
 		</div>