Add abuse protection to the ROI calculator form

The calculator endpoint was open to anyone, and it has been collecting
spam submissions. Add a layered guard that runs before anything is
written to the DB or sent to the AI provider:

- Cloudflare Turnstile CAPTCHA, verified server-side (active when
  TURNSTILE_* keys are configured; fails closed if unverifiable)
- Honeypot field that only bots fill in
- Signed, single-use form token enforcing a 4s minimum fill time
- Per-IP rate limiting (5 per 10 min, 25 per 24h)

Layers 2-4 need no configuration and work on their own, so submissions
are throttled immediately; adding Turnstile keys upgrades it to a full
challenge. Blocked submissions now stop the flow client-side instead of
silently showing a result.

Also sets `trust proxy` for correct client IPs behind nginx, caps the
JSON body at 32kb, and fixes a latent ReferenceError in the
"AI not configured" branch that called saveCalcSubmission() before the
variables it closes over were declared.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server.js

@@ -16,6 +16,8 @@ const express = require('express');
 const Database = require('better-sqlite3');
 const OpenAI   = require('openai');
 
+const security = require('./security');
+
 // ── Config ──────────────────────────────────────────────
 const PORT    = process.env.PORT || 3000;
 
@@ -107,9 +109,22 @@ const getAllLeads = db.prepare(`
 
 // ── App ───────────────────────────────────────────────────
 const app = express();
-app.use(express.json());
+app.set('trust proxy', 1);            // behind nginx — needed for correct client IPs
+app.use(express.json({ limit: '32kb' }));
 app.use(express.static(__dirname));   // serve the marketing site
 
+// GET /api/form-config  — public config the forms need (CAPTCHA site key)
+app.get('/api/form-config', (_req, res) => {
+  res.set('Cache-Control', 'no-store');
+  res.json(security.publicConfig());
+});
+
+// GET /api/form-token  — single-use, signed token proving the form was loaded
+app.get('/api/form-token', (_req, res) => {
+  res.set('Cache-Control', 'no-store');
+  res.json({ token: security.issueFormToken() });
+});
+
 // POST /api/leads  — capture a new preview sign-up
 app.post('/api/leads', (req, res) => {
   const { firstName, lastName, email, orgName, state, role, unitCount, betaInterest, source } = req.body ?? {};
@@ -193,9 +208,11 @@ app.post('/api/calculate', async (req, res) => {
     }
   }
 
-  if (!aiClient) {
-    saveCalcSubmission(null);
-    return res.status(503).json({ error: 'AI service not configured.' });
+  // ── Abuse protection: CAPTCHA, honeypot, form token, rate limit ──
+  // Runs before anything is written to the DB or sent to the AI provider.
+  const guard = await security.guardSubmission(req);
+  if (!guard.ok) {
+    return res.status(guard.status).json({ error: guard.error, blocked: true });
   }
 
   const {
@@ -207,6 +224,11 @@ app.post('/api/calculate', async (req, res) => {
     return res.status(400).json({ error: 'homesites and annualIncome are required.' });
   }
 
+  if (!aiClient) {
+    saveCalcSubmission(null);
+    return res.status(503).json({ error: 'AI service not configured.' });
+  }
+
   const fmt          = n => '$' + Math.round(n).toLocaleString();
   const typeLabel    = { sfh: 'single-family home', townhomes: 'townhome', condos: 'condo', mixed: 'mixed-use' }[propertyType] || '';
   const freqDivisor  = { monthly: 12, quarterly: 4, annually: 1 }[paymentFreq] || 12;