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/htmx/updated_row.html

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+{% comment %}
+FSM Updated Row Partial Template
+
+Generic template for rendering an updated table row after an FSM transition.
+Used as the default response template for FSMTransitionView.
+
+Required context:
+  - object: The FSM-enabled model instance
+  - user: The current user (usually request.user)
+
+Optional context:
+  - transition_success: Whether a transition just succeeded
+  - success_message: Success message to display (handled by toast)
+  - show_actions: Whether to show action buttons (defaults to true)
+  - row_class: Additional CSS classes for the row
+{% endcomment %}
+{% load fsm_tags %}
+
+<tr id="{{ object|default_target_id }}"
+    class="{% if transition_success %}animate-flash-success{% endif %} {{ row_class|default:'' }} border-b border-gray-200/50 dark:border-gray-700/50 hover:bg-gray-50 dark:hover:bg-gray-800/50 transition-colors">
+
+    <!-- Status Cell -->
+    <td class="px-4 py-3">
+        <span class="status-badge inline-flex items-center gap-1.5 px-2.5 py-1 text-xs font-medium rounded-full
+            {% with status=object|get_state_value %}
+            {% if status == 'PENDING' %}
+            bg-yellow-100 text-yellow-800 dark:bg-yellow-900/30 dark:text-yellow-300
+            {% elif status == 'APPROVED' %}
+            bg-green-100 text-green-800 dark:bg-green-900/30 dark:text-green-300
+            {% elif status == 'REJECTED' %}
+            bg-red-100 text-red-800 dark:bg-red-900/30 dark:text-red-300
+            {% elif status == 'ESCALATED' %}
+            bg-orange-100 text-orange-800 dark:bg-orange-900/30 dark:text-orange-300
+            {% elif status == 'IN_PROGRESS' or status == 'PROCESSING' %}
+            bg-blue-100 text-blue-800 dark:bg-blue-900/30 dark:text-blue-300
+            {% elif status == 'COMPLETED' or status == 'RESOLVED' %}
+            bg-green-100 text-green-800 dark:bg-green-900/30 dark:text-green-300
+            {% else %}
+            bg-gray-100 text-gray-800 dark:bg-gray-900/30 dark:text-gray-300
+            {% endif %}
+            {% endwith %}">
+            {% with status=object|get_state_value %}
+            <i class="fas fa-{% if status == 'PENDING' %}clock{% elif status == 'APPROVED' %}check{% elif status == 'REJECTED' %}times{% elif status == 'ESCALATED' %}exclamation{% elif status == 'IN_PROGRESS' or status == 'PROCESSING' %}spinner{% elif status == 'COMPLETED' or status == 'RESOLVED' %}check-circle{% else %}circle{% endif %}"></i>
+            {% endwith %}
+            {{ object|get_state_display }}
+        </span>
+    </td>
+
+    <!-- Object Info Cell -->
+    <td class="px-4 py-3">
+        <div class="text-sm font-medium text-gray-900 dark:text-gray-100">
+            {{ object }}
+        </div>
+        {% if object.created_at %}
+        <div class="text-xs text-gray-500 dark:text-gray-400">
+            {{ object.created_at|date:"M d, Y H:i" }}
+        </div>
+        {% endif %}
+    </td>
+
+    <!-- Actions Cell -->
+    {% if show_actions|default:True %}
+    <td class="px-4 py-3 text-right">
+        {% include 'htmx/state_actions.html' with object=object user=user button_size='sm' inline=True %}
+    </td>
+    {% endif %}
+</tr>
+
+{% comment %}
+CSS for flash animation - add to your CSS file or include inline:
+
+@keyframes flash-success {
+    0%, 100% { background-color: transparent; }
+    50% { background-color: rgba(34, 197, 94, 0.2); }
+}
+
+.animate-flash-success {
+    animation: flash-success 1s ease-in-out;
+}
+{% endcomment %}