docs(retake): Add documentation for retake nodes

doc/architecture/nodes/RetakeScheduler.md

@@ -0,0 +1,47 @@
+# RetakeScheduler (`assignments::one::retake_scheduler`)
+
+## Overview
+The `RetakeScheduler` node automatically identifies students who have failed courses and schedules retake exams for them. It periodically queries the database for failing students and sends retake action requests to the `RetakeGradeDeterminator` node to initiate the retake process.
+
+#### Implementation Details
+
+**Parameters**
+
+- **`retake_check_interval_sec`** (int, default: 120): Time interval in seconds between checks for failing students requiring retakes.
+
+**Constructor**
+```cpp
+RetakeScheduler(std::unique_ptr<DatabaseManager> db_manager = nullptr)
+```
+- Initializes ROS2 node with name `retake_scheduler`
+- Sets up `DatabaseManager` (optional injection for testing)
+- Creates ROS2 action client for retake action communication
+- Initializes wall timer for periodic failing student checks
+
+**Core Functions**
+
+**`void check_failing_students()`**
+- Timer callback function executed periodically based on `retake_check_interval_sec`
+- Queries database for all failed course results
+- Iterates through failing students and initiates retake requests
+
+**`void send_retake_request(StudentCourse student_course_combo)`**
+- Creates retake action goal with student and course information
+- Sends asynchronous retake goal to `RetakeGradeDeterminator`
+
+**`void cancel_retake_request()`**
+- Cancels all active retake goals
+- Provides emergency stop functionality for retake processing
+
+**Action Client Callbacks**
+
+**`void request_response_callback(const rclcpp_action::ClientGoalHandle<RetakeAction>::SharedPtr &goal_handle)`**
+- Handles retake action server responses
+- Logs goal acceptance or rejection status
+- Provides feedback on retake request processing state
+
+**`void request_result_callback(const rclcpp_action::ClientGoalHandle<RetakeAction>::WrappedResult &result)`**
+- Processes final retake action results
+- Logs appropriate messages based on retake completion status
+
+**`void request_feedback_callback(rclcpp_action::ClientGoalHandle<RetakeAction>::SharedPtr goal_handle, const std::shared_ptr<const RetakeAction::Feedback> feedback)`**