perf(AssessmentStatistics): add lazyloading for Statistics page data

[craigtonlian]

Problem

The /main_statistics route at the assessments#main_statistics fetches all statistics-related data at once. This includes:

• Assessment statistics
• Submission statistics
• Ancestor info

Note: Live feedback data has already been separated.

This design results in unnecessary data fetching, especially when users visit tabs that do not require certain datasets.

Changes

We’ve separated the following data concerns into individual endpoints:

• /assessment_statistics
• /submission_statistics
• /ancestor_info

This allows for on-demand fetching, triggered only when the user navigates to the tab that needs the corresponding data.

Tab-to-Data Mapping

Below is the breakdown of which data each tab requires:

Tab	Required Data
StudentGradesPerQuestionTable	Assessment, Submissions
StudentAttemptCountTable	Assessment, Submissions
MainGradesChart	Submissions
MainSubmissionTimeAndGradeStatistics	Submissions
DuplicationHistoryStatistics	Ancestor
LiveFeedbackStatistics	Assessment, Live Feedback

Each tab first checks the Redux store for cached data and fetches only if necessary.

Design Considerations

While we considered reusing existing endpoints (submissions#index, assessments#show), we opted to split out dedicated statistics endpoints for the following reasons:

• Course::Statistics::AssessmentsController uses .unscope to bypass default scopes, which could introduce side effects if added to #show.
• Specific associations are required for statistics (e.g. includes(programming_questions: [:language])).
• Keeps controllers lean and prevents unnecessary data loading in CRUD endpoints.
• Course::Assessment::AssessmentsController has concerns for assessment management and external service integration, while Course::Statistics::AssessmentsController has concerns that handle data aggregation.