feat(infra) : add nginx-proxy.conf with maintenance mode

- infra/prod/nginx-proxy.conf: reverse proxy with maintenance file check
- Updated deployment doc with maintenance mode instructions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

infra/prod/nginx-proxy.conf

@@ -0,0 +1,31 @@
+server {
+    listen 80;
+    listen [::]:80;
+    server_name coltura.malio-dev.fr;
+
+    root /var/www/coltura/public;
+
+    # Maintenance mode
+    if (-f /var/www/coltura/maintenance.on) {
+        return 503;
+    }
+
+    error_page 503 @maintenance;
+
+    location @maintenance {
+        rewrite ^(.*)$ /maintenance.html break;
+    }
+
+    location = /maintenance.html {
+        internal;
+    }
+
+    location / {
+        proxy_pass http://127.0.0.1:8083;
+        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 X-Forwarded-Proto $scheme;
+        client_max_body_size 55m;
+    }
+}