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>

package.json

@@ -9,6 +9,8 @@
   },
   "dependencies": {
     "better-sqlite3": "^9.4.3",
-    "express": "^4.18.3"
+    "dotenv": "^17.3.1",
+    "express": "^4.18.3",
+    "openai": "^6.27.0"
   }
 }