fix(config): Add IPv6 listening to nginx for healthcheck compatibility

The Docker healthcheck uses wget with localhost, which resolves to ::1
(IPv6) first. Without IPv6 listening, the healthcheck fails and Traefik
won't route traffic to the container.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: enko

docker/nginx.conf.template

@@ -46,6 +46,7 @@ http {
 
     server {
         listen 80;
+        listen [::]:80;
         server_name _;
 
         # Security headers (Traefik handles HTTPS, so these are internal)

docker/nginx.dev.conf

@@ -39,6 +39,7 @@ http {
 
     server {
         listen 80;
+        listen [::]:80;
         server_name _;
 
         # Increase client body size for vCard uploads

docker/nginx.multiservice.conf.template

@@ -45,6 +45,7 @@ http {
 
     server {
         listen 80;
+        listen [::]:80;
         server_name _;
 
         # Security headers

docker/nginx.prod.conf

@@ -44,6 +44,7 @@ http {
 
     server {
         listen 80;
+        listen [::]:80;
         server_name _;
 
         # Security headers (Traefik handles HTTPS, so these are internal)