web-app-pretix: fix healthcheck and allowed hosts

- Add Host header to curl healthcheck when container_hostname is defined
- Use PRETIX_PRETIX_ALLOWED_HOSTS to fix Django 400 Bad Request during healthcheck
- Centralize PRETIX_HOSTNAME from container_hostname var
- Add Redis broker/result backend config for Celery

See: https://chatgpt.com/share/68b59c42-c0fc-800f-9bfb-f1137c59b3de

roles/web-app-pretix/templates/env.j2

@@ -1,5 +1,6 @@
 ## Pretix core
 PRETIX_PRETIX_INSTANCE_NAME="{{ PRIMARY_DOMAIN | upper }} Tickets"
+PRETIX_PRETIX_ALLOWED_HOSTS="{{ PRETIX_HOSTNAME }},127.0.0.1,localhost"
 PRETIX_PRETIX_URL="{{ PRETIX_URL }}"
 PRETIX_PRETIX_AUTH_BACKENDS="pretix.base.auth.NativeAuthBackend{% if PRETIX_OIDC_ENABLED %},pretix_oidc.auth.OIDCAuthBackend{% endif %}"
 
@@ -16,12 +17,16 @@ PRETIX_DATABASE_PORT="{{ database_port }}"
 PRETIX_WEB_CONCURRENCY="{{ POSTGRES_ALLOWED_AVG_CONNECTIONS }}"
 PRETIX_WORKER_THREADS="{{ (POSTGRES_ALLOWED_AVG_CONNECTIONS | int // 2 ) }}"
 
+{% if PRETIX_REDIS_ENABLED %}
 ## Redis
-PRETIX_REDIS_LOCATION="redis://redis:6379/1"
 PRETIX_REDIS_SESSIONS="true"
+PRETIX_REDIS_LOCATION="redis://redis:6379/{{ PRETIX_REDIS_CACHE_DB }}"
+CELERY_BROKER_URL="redis://redis:6379/{{ PRETIX_REDIS_BROKER_DB }}"
+CELERY_RESULT_BACKEND="redis://redis:6379/{{ PRETIX_REDIS_RESULT_DB }}"
+{% endif %}
 
-## OIDC (plugin)
 {% if PRETIX_OIDC_ENABLED %}
+## OIDC (plugin)
 PRETIX_OIDC_TITLE="{{ PRETIX_OIDC_LABEL | replace('\"','\\\"') }}"
 PRETIX_OIDC_ISSUER="{{ PRETIX_OIDC_ISSUER }}"
 PRETIX_OIDC_AUTHORIZATION_ENDPOINT="{{ PRETIX_OIDC_AUTH_URL }}"