Add pretix-oidc plugin for Keycloak SSO integration

Enables OIDC authentication against Keycloak and automatic team assignment
via claim-based rules (pretix_scanner, pretix_shop, pretix_admin).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -62,7 +62,8 @@ RUN pip3 install -U \
     cd /pretix && \
     PRETIX_DOCKER_BUILD=TRUE pip3 install \
         -e ".[memcached]" \
-        gunicorn django-extensions ipython && \
+        gunicorn django-extensions ipython \
+        pretix-oidc && \
     rm -rf ~/.cache/pip
 
 RUN chmod +x /usr/local/bin/pretix && \