docs(retake): Add documentation for retake nodes

doc/architecture/nodes/RetakeGradeDeterminator.md

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+# RetakeGradeDeterminator (`assignments::one::retake_grade_determinator`)
+
+## Overview
+The `RetakeGradeDeterminator` node handles the processing of retake exams for students who have
+failed courses. It provides a ROS2 action server that accepts retake requests, manages exam result
+collection for retake scenarios, and coordinates with the grade calculator service to determine
+final retake grades.
+
+#### Implementation Details
+
+**Parameters**
+
+- **`grade_collection_amount`** (int, default: 5): Number of exam results required before triggering grade calculation for a retake student-course combination.
+
+**Constructor**
+```cpp
+RetakeGradeDeterminator(std::unique_ptr<DatabaseManager> db_manager = nullptr)
+```
+- Initializes ROS2 node with name `retake_grade_determinator`
+- Sets up `DatabaseManager` (optional injection for testing)
+- Creates publisher for student course management
+- Subscribes to exam results topic with retake-specific callback
+- Initializes service client for grade calculation
+- Creates ROS2 action server for retake requests with reentrant callback group
+
+**Core Functions**
+
+**`void exam_results_callback(const g2_2025_interfaces::msg::Exam::SharedPtr msg)`**
+- Only processes exam results when `retake_allowed_` flag is true
+- Triggers grade calculation request when threshold is met
+
+**`void grade_calculator_request(StudentCourse combo)`**
+- Waits for grade calculator service to be available
+- Sends async request with collected retake exam grades
+
+**`void grade_calculator_response(rclcpp::Client<g2_2025_interfaces::srv::Exams>::SharedFuture future, StudentCourse studentCourseCombo)`**
+- Clears collected exam data from internal map
+- Updates retake status in database for the student-course combination
+- Publishes final student message to ROS2 topic with timestamp
+- Stores final course result in database with retake flag set to true
+
+**Action Server Callbacks**
+
+**`rclcpp_action::GoalResponse goal_callback(const rclcpp_action::GoalUUID& uuid, std::shared_ptr<const g2_2025_interfaces::action::Retake::Goal> goal)`**
+- Accepts retake goal requests for specific student-course combinations
+- Logs received retake requests with student and course information
+- Returns `ACCEPT_AND_EXECUTE` for all valid requests
+
+**`rclcpp_action::CancelResponse cancel_callback(const std::shared_ptr<rclcpp_action::ServerGoalHandle<g2_2025_interfaces::action::Retake>> goal_handle)`**
+- Handles retake action cancellation requests
+- Returns `ACCEPT` for all cancellation requests
+
+**`void spawn_callback_thread(const std::shared_ptr<rclcpp_action::ServerGoalHandle<g2_2025_interfaces::action::Retake>> goal_handle)`**
+- Creates detached thread for asynchronous retake action execution
+- Ensures non-blocking retake processing
+
+**`void async_execute_callback_thread(const std::shared_ptr<rclcpp_action::ServerGoalHandle<g2_2025_interfaces::action::Retake>> goal_handle)`**
+- Enables retake exam processing by setting `retake_allowed_` flag
+- Publishes student enrollment message to trigger exam generation
+- Creates and returns successful action result

doc/architecture/nodes/RetakeScheduler.md

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+# 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)`**