docs: add nginx deploy instructions and fix Authorization header config

- Add proxy_set_header Authorization to /api/ location (required or token is stripped)
- Add Step 11 to README explaining how to install the nginx config

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

nginx.conf

@@ -2,13 +2,14 @@ server {
     listen 80;
     server_name _;
 
-    # API requests → backend
+    # API requests → backend (Authorization header must be explicit or it gets stripped)
     location /api/ {
         proxy_pass http://localhost:5000;
         proxy_http_version 1.1;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header Authorization $http_authorization;
     }
 
     # Socket.IO → backend (WebSocket upgrade)