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/models/tos.js

@@ -0,0 +1,34 @@
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+const Table = require('.');
+
+// Terms-of-Service text, editable by an admin at runtime (see routes/tos.js
+// + the Dashboard's "Terms of Service" card) instead of being baked into the
+// repo. A singleton row -- always keyed 'current' -- rather than a UUID like
+// the other Redis models here, since there's only ever one live ToS.
+class Tos extends Table {
+	static _key = 'name';
+	static _keyMap = {
+		name:       {default: 'current', type: 'string'},
+		content:    {isRequired: true,   type: 'string'},
+		updated_by: {isRequired: true,   type: 'string'},
+		updated_on: {default: () => Date.now()},
+	};
+
+	// Fetch the live row, seeding it from the bundled tos.md template the
+	// first time this is ever called on a deployment (so upgrading an
+	// existing install doesn't start with a blank ToS).
+	static async getCurrent() {
+		try {
+			return await this.get('current');
+		} catch (error) {
+			const content = fs.readFileSync(path.join(__dirname, '../../tos.md'), 'utf8');
+			return this.create({name: 'current', content, updated_by: 'system'});
+		}
+	}
+}
+Tos.register();
+
+module.exports = {Tos};