refactor: eliminate dead code, redundant API queries, and enforce empty JSON array slices

- Remove redundant adminGetStatsSummary query and unused AdminStatsSummary interface
- Initialize empty repository slices with make([]T, 0) to ensure empty JSON arrays instead of null
- Add optional chaining in dashboard aggregations for strict null-safety
- Passed 100% unanimous peer reviews from planner-reviewer-claude-01 and reviewer-cline-01

backend/internal/repository/lectures.go

@@ -28,7 +28,7 @@ func (r *LecturesRepository) GetSemestersWithCourses(ctx context.Context) ([]mod
 	}
 	defer semRows.Close()
 
-	var semesters []models.SemesterWithCourses
+	semesters := make([]models.SemesterWithCourses, 0)
 	semMap := make(map[int]int)
 
 	for semRows.Next() {