feat(signup): community guidelines agreement and policies routes

Introduces /community-guidelines and /policies/{privacy,terms,[slug]} pages,
swaps the signup/invite checkbox from agreedToTerms to agreedToGuidelines,
adds Member.agreement.acceptedAt, and stamps the field when a Helcim
customer is created.

server/models/member.js

@@ -83,6 +83,10 @@ const memberSchema = new mongoose.Schema({
   inviteEmailSent: { type: Boolean, default: false },
   inviteEmailSentAt: Date,
 
+  agreement: {
+    acceptedAt: Date,
+  },
+
   // Magic link single-use enforcement
   magicLinkJti: String,
   magicLinkJtiUsed: { type: Boolean, default: false },