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/config/django/base.py

@@ -148,6 +148,7 @@ if TEMPLATES_ENABLED:
                     "django.contrib.auth.context_processors.auth",
                     "django.contrib.messages.context_processors.messages",
                     "apps.moderation.context_processors.moderation_access",
+                    "apps.core.context_processors.fsm_context",
                 ]
             },
         }
@@ -166,6 +167,7 @@ else:
                     "django.contrib.auth.context_processors.auth",
                     "django.contrib.messages.context_processors.messages",
                     "apps.moderation.context_processors.moderation_access",
+                    "apps.core.context_processors.fsm_context",
                 ]
             },
         }