Add test utilities and state machine diagrams for FSM models

- Introduced reusable test utilities in `backend/tests/utils` for FSM transitions, HTMX interactions, and common scenarios.
- Added factory functions for creating test submissions, parks, rides, and photo submissions.
- Implemented assertion helpers for verifying state changes, toast notifications, and transition logs.
- Created comprehensive state machine diagrams for all FSM-enabled models in `docs/STATE_DIAGRAMS.md`, detailing states, transitions, and guard conditions.

backend/thrillwiki/urls.py

@@ -96,6 +96,8 @@ urlpatterns = [
     path("user/", accounts_views.user_redirect_view, name="user_redirect"),
     # Moderation URLs - placed after other URLs but before static/media serving
     path("moderation/", include("apps.moderation.urls", namespace="moderation")),
+    # Core app URLs (FSM transitions, entity search)
+    path("core/", include("apps.core.urls", namespace="core")),
     path(
         "env-settings/",
         views.environment_and_settings_view,