feat(pretix): add ansible role and playbook

roles/pretix/README.md

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+# `finallycoffee.services.pretix` ansible role
+
+Deploy [pretix](https://pretix.eu) using ansible. Note that this
+role does not configure pretix beyond its own configuration file,
+and requires changing a default admin password after a successful
+installation.
+
+## Configuration
+
+For all available configuration options, see [`defaults/main/config.yml`](defaults/main/config.yml)
+and other supporting files in the [`defaults/main/`](defaults/main/) folder.
+
+To add custom configuration to pretix, populate them in `pretix_config`,
+where they will be (recusively) merged into the default configuration.
+
+### Required
+
+- `pretix_domain`: domain of the pretix instance
+- `pretix_postgresql_password`: password for the (default: postgresql) database
+- `pretix_config_redis_location`: connection string for the main pretix redis database
+- `pretix_config_celery_backend`: connection string for the celery backend, can be a (different!) redis database
+- `pretix_config_celery_broker`: connection string for the celery broker, can be a (yet another different) redis database
+
+For examples on how a redis server (like valkey) can be configured
+for redis, see [`playbooks/pretix.yml`](../../playbooks/pretix.yml).
+
+### Mailing
+
+Set up mails in pretix by populating the following variables:
+- `pretix_config_mail_host`: domain/IP and optional port of the SMTP server
+- `pretix_config_mail_user`: SMTP user to authenticate
+- `pretix_config_mail_password`: password for the SMTP user
+
+### Plugins
+
+To install more plugins, list the wanted `pypi` packages as a list in
+`pretix_plugins`. They will be installed in the created virtualenv, and migrations and an asset rebuild will be automatically started.
+
+If your plugin requires custom configuration (f.ex.: `pretix-oidc`),
+add the configuration into `pretix_config`.
+
+## Troubleshooting
+
+### virtualenv
+
+By default, the virtualenv is located in `/var/lib/pretix/virtualenv`.
+This can be controlled by setting `pretix_virtualenv_dir`.
+
+NOTE: To fix a broken virtualenv, try setting `pretix_virtualenv_state` to `forcereinstall` (see
+[`ansible.builtin.pip` on docs.ansible.com](https://docs.ansible.com/ansible/latest/collections/ansible/builtin/pip_module.html)).
+
+NOTE: To install pip packages or execute migrations in the virtualenv, ansible
+needs to become the unprivilated `pretix_user` (default: `pretix`). This might
+require having the `acl` system package installed.