feat: async AI calls, 10-min timeout, and failure messaging

- Make all AI endpoints (health scores + investment recommendations)
  fire-and-forget: POST returns immediately, frontend polls for results
- Extend AI API timeout from 2-5 min to 10 min for both services
- Add "last analysis failed — showing cached data" message to the
  Investment Recommendations panel (matches health score widgets)
- Add status/error_message columns to ai_recommendations table
- Remove nginx AI timeout overrides (no longer needed)
- Users can now navigate away during AI processing without interruption

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

nginx/production.conf

@@ -40,20 +40,8 @@ server {
         proxy_send_timeout 15s;
     }
 
-    # AI endpoints → longer timeouts
-    location /api/investment-planning/recommendations {
-        proxy_pass http://backend;
-        proxy_read_timeout 180s;
-        proxy_connect_timeout 10s;
-        proxy_send_timeout 30s;
-    }
-
-    location /api/health-scores/calculate {
-        proxy_pass http://backend;
-        proxy_read_timeout 180s;
-        proxy_connect_timeout 10s;
-        proxy_send_timeout 30s;
-    }
+    # AI endpoints now return immediately (async processing in background)
+    # No special timeout overrides needed
 
     # --- Static frontend → built React assets ---
     location / {