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/tests/utils/__init__.py

@@ -0,0 +1,28 @@
+"""
+Test utilities for ThrillWiki.
+
+This package provides reusable test utilities, helpers, and fixtures
+for testing FSM transitions, HTMX interactions, and other common scenarios.
+"""
+
+from .fsm_test_helpers import (
+    create_test_submission,
+    create_test_park,
+    create_test_ride,
+    assert_status_changed,
+    assert_state_log_created,
+    assert_toast_triggered,
+    wait_for_htmx_swap,
+    verify_transition_buttons_visible,
+)
+
+__all__ = [
+    "create_test_submission",
+    "create_test_park",
+    "create_test_ride",
+    "assert_status_changed",
+    "assert_state_log_created",
+    "assert_toast_triggered",
+    "wait_for_htmx_swap",
+    "verify_transition_buttons_visible",
+]