Fix unnecessary Study Plan fetch and backend 500 when no plan exists #110

Summary

ClassDetailPage fetches the study plan on every load regardless of whether an exam date is set. When no plan has been created yet, the backend throws a generic RuntimeException (→ HTTP 500) instead of a clean 404.

Steps to Reproduce

1. Navigate to any ClassDetailPage for a class without an exam date set
2. Observe the network request to GET /api/classes/{classId}/study-plan
3. Observe the resulting 500 error in the backend logs

Expected Behaviour

• The study plan API call is only made if the class has an exam date set (study plans are only relevant with an exam date).
• When a class has an exam date but no study plan has been created yet, GET /api/classes/{classId}/study-plan returns HTTP 404 — not 500 — because the absence of a plan is a valid, expected state.

Actual Behaviour

• useStudyPlan(id) is called unconditionally at ClassDetailPage.tsx:61, triggering an unnecessary API request even when no exam date exists.
• StudyPlanService.getStudyPlan() (StudyPlanService.java:108) throws new RuntimeException("No study plan found for this class") when no plan exists, which Spring maps to HTTP 500.

Environment

Affected Module(s)

• minerva-api
• minerva-hub

Severity

• Medium — feature degraded, workaround exists (the frontend already handles isError gracefully and doesn't retry on 404/500, but the 500 pollutes logs and the unnecessary call wastes a round-trip)

Suspected Cause

• Frontend (ClassDetailPage.tsx:61): useStudyPlan(id) has no guard for classData.examDate. The condition canCreateStudyPlan already checks classData.examDate at line 252, but the fetch itself does not.
• Backend (StudyPlanService.java:108): Uses orElseThrow(() -> new RuntimeException(...)) instead of a typed NotFoundException / ResponseStatusException(HttpStatus.NOT_FOUND).

Proposed Fix

1. minerva-hub — ClassDetailPage.tsx: Pass the classId to the hook only when the exam date is set:

   // only fetch when exam date is present
   const { data: studyPlan } = useStudyPlan(classData?.examDate ? id : undefined);

2. minerva-api — StudyPlanService.java:108: Replace the generic RuntimeException with a proper not-found exception so the endpoint returns HTTP 404 instead of 500.

Logs / Stack Trace

RuntimeException: No study plan found for this class
  at StudyPlanService.getStudyPlan(StudyPlanService.java:108)