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/config/main.yml

@@ -2,16 +2,15 @@ credentials: {}
 docker:
   services:
     redis:
-      enabled:  true
+      enabled:      true
     database:
-      enabled:  true
-    application:
-      image:    pretix/standalone
-      version:  stable
-      name:     pretix
+      enabled:      true
+    pretix:
+      image:        pretix/standalone
+      version:      stable
+      name:         pretix
   volumes:
-    data:       "pretix_data"
-    config:     "pretix_config"
+    data:           "pretix_data"
 features:
   matomo:           true
   css:              true