web-app-pretix: fully implemented role

Summary:
- Replace draft with complete README (features, resources, credits).
- Remove obsolete Todo.md.
- Switch to custom image tag (PRETIX_IMAGE_CUSTOM) and install 'pretix-oidc' in Dockerfile.
- Drop unused 'config' volume; keep persistent 'data' only.
- Rename docker-compose service from 'application' to 'pretix' and use container_port.
- Use standard depends_on include for DB/Redis (dmbs_excl).
- Align vars to docker.services.pretix.* (image/version/name); add PRETIX_IMAGE_CUSTOM.

Breaking:
- Service key changed to 'pretix' under docker.services.
- 'config' volume removed from compose.

Status:
- Pretix role is now fully implemented and production-ready.

Reference:
- Conversation: https://chatgpt.com/share/68b1cb34-b7dc-800f-8b39-c183124972f2

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

@@ -1,4 +1,3 @@
-ARG PRETIX_BASE_IMAGE={{ PRETIX_IMAGE }}:{{ PRETIX_VERSION }}
-FROM ${PRETIX_BASE_IMAGE}
+FROM {{ PRETIX_IMAGE }}:{{ PRETIX_VERSION }}
 # Install OIDC auth plugin for Pretix
 RUN python -m pip install --no-cache-dir "pretix-oidc=={{ PRETIX_OIDC_PLUGIN_VERSION }}"