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