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>

styles.css

@@ -821,6 +821,23 @@ a.feature-card:hover { transform: translateY(-4px); box-shadow: var(--shadow-lg)
 }
 .input-prefix-wrap input { padding-left: 28px; }
 .calc-error { color: var(--red); font-size: 13px; margin: 8px 0; font-weight: 500; }
+
+/* Honeypot — visually and semantically hidden, but still fillable by bots.
+   Deliberately not `display:none`, which the better bots skip. */
+.calc-hp {
+  position: absolute !important;
+  left: -9999px;
+  top: -9999px;
+  width: 1px;
+  height: 1px;
+  overflow: hidden;
+  opacity: 0;
+  pointer-events: none;
+}
+
+/* CAPTCHA widget slot — collapsed until a challenge is actually rendered */
+.calc-captcha:empty { display: none; }
+.calc-captcha { margin: 12px 0 0; display: flex; justify-content: center; }
 .calc-submit-btn { width: 100%; justify-content: center; margin-top: 16px; }
 .calc-fine {
   font-size: 11px;