Add OpenAI-compatible AI investment advisor to benefit calculator

- server.js: add /api/calculate endpoint using openai SDK with configurable
  AI_API_URL, AI_API_KEY, AI_MODEL, AI_DEBUG env vars (works with any
  OpenAI-compatible provider: NVIDIA NIM, Together AI, Groq, Ollama, etc.)
- app.js: make calculator submit handler async; call /api/calculate with
  graceful fallback to client-side generated text if AI is unavailable
- package.json: add openai and dotenv dependencies
- AI_SETUP.md: rewrite to document new unified env var config with provider examples

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.js

@@ -52,7 +52,7 @@
     close();
   });
 
-  submitBtn?.addEventListener('click', () => {
+  submitBtn?.addEventListener('click', async () => {
     const homesites    = parseFloat(document.getElementById('calcHomesites').value)    || 0;
     const propertyType = document.getElementById('calcPropertyType').value;
     const annualIncome = parseFloat(document.getElementById('calcAnnualIncome').value) || 0;
@@ -116,7 +116,22 @@
       ai += `This would represent entirely new interest income for your community at no additional risk.`;
     }
 
-    document.getElementById('calcAiText').textContent = ai;
+    // ── AI recommendation — call server; fall back to client-side text ──
+    try {
+      const aiRes = await fetch('/api/calculate', {
+        method:  'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body:    JSON.stringify({ homesites, propertyType, annualIncome, paymentFreq, reserveFunds, interest2025 }),
+      });
+      if (aiRes.ok) {
+        const { recommendation } = await aiRes.json();
+        document.getElementById('calcAiText').textContent = recommendation;
+      } else {
+        document.getElementById('calcAiText').textContent = ai;
+      }
+    } catch (_) {
+      document.getElementById('calcAiText').textContent = ai;
+    }
 
     // ── Animate the main number ──
     animateValue(document.getElementById('resultAmount'), 0, totalPotential);