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/vars/main.yml

@@ -2,12 +2,14 @@
 application_id:                 "web-app-pretix"
 database_type:                  "postgres"
 container_port:                 80
+container_hostname:             "{{ domains | get_domain(application_id) }}"
 
-# URLs
+# Pretix
+## URLs
 PRETIX_URL:                     "{{ domains | get_url(application_id, WEB_PROTOCOL) }}"
-PRETIX_HOSTNAME:                "{{ domains | get_domain(application_id) }}"
+PRETIX_HOSTNAME:                "{{ container_hostname }}"
 
-# OIDC (mirrors GitLab’s pattern)
+## OIDC (mirrors GitLab’s pattern)
 PRETIX_OIDC_ENABLED:            "{{ applications | get_app_conf(application_id, 'features.oidc') }}"
 PRETIX_OIDC_LABEL:              "{{ OIDC.BUTTON_TEXT }}"
 PRETIX_OIDC_CLIENT_ID:          "{{ OIDC.CLIENT.ID }}"
@@ -22,7 +24,13 @@ PRETIX_OIDC_SCOPES:             "openid,email,profile"
 # Use Keycloak username claim by default (plugin default is 'sub')
 PRETIX_OIDC_UNIQUE_ATTRIBUTE:   "{{ OIDC.ATTRIBUTES.USERNAME }}"
 
-# Docker
+## Redis
+PRETIX_REDIS_ENABLED:           "{{ applications | get_app_conf(application_id, 'docker.services.redis.enabled') }}"
+PRETIX_REDIS_CACHE_DB:          1
+PRETIX_REDIS_BROKER_DB:         0
+PRETIX_REDIS_RESULT_DB:         2
+
+## Docker
 PRETIX_IMAGE_CUSTOM:            "pretix_custom"
 PRETIX_IMAGE:                   "{{ applications | get_app_conf(application_id, 'docker.services.pretix.image') }}"
 PRETIX_VERSION:                 "{{ applications | get_app_conf(application_id, 'docker.services.pretix.version') }}"