feat: Implement grading functionality for handwritten answers
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6f5ba40
- Added `grade` method to `AnswerScans` class for initiating grading actions.
- Enhanced error handling for grading limits and busy states in the grading process.
- Updated `public_result` function to clean up grading-related fields from results.
- Introduced new grading verification tests to ensure correct behavior of grading API.
- Created `AnswerGrader` class to handle grading logic and validation of assessment results.
- Added database migration for `math_answer_grades` table to store grading results.
- Updated front-end JavaScript to handle grading states and display results appropriately.
- Enhanced CSS styles for grading feedback display.
- Implemented comprehensive tests for grading functionality, including edge cases and error handling.
