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/templates/base/base.html

@@ -136,6 +136,27 @@
     <!-- Custom JavaScript -->
     <script src="{% static 'js/main.js' %}"></script>
     <script src="{% static 'js/alerts.js' %}"></script>
+    <script src="{% static 'js/fsm-transitions.js' %}"></script>
+
+    <!-- Handle HX-Trigger headers for toast notifications -->
+    <script>
+    document.body.addEventListener('htmx:afterOnLoad', function(evt) {
+        const triggerHeader = evt.detail.xhr.getResponseHeader('HX-Trigger');
+        if (triggerHeader) {
+            try {
+                const triggers = JSON.parse(triggerHeader);
+                if (triggers.showToast && Alpine && Alpine.store('toast')) {
+                    Alpine.store('toast')[triggers.showToast.type || 'info'](
+                        triggers.showToast.message,
+                        triggers.showToast.duration
+                    );
+                }
+            } catch (e) {
+                // Ignore parsing errors for non-JSON triggers
+            }
+        }
+    });
+    </script>
     {% block extra_js %}{% endblock %}
   </body>
 </html>