Replace single-letter lambda parameters with descriptive names

[Copilot]

Lambda parameters throughout the codebase use single letters (s, c, r, e) making LINQ queries difficult to scan and understand at a glance.

Changes

• Repository layer: Replaced all single-letter lambda parameters in LINQ queries
  • s → student in StudentRepository
  • c → course in CourseRepository
  • r → registration in RegistrationRepository
• DbContext: Updated Entity Framework model configuration lambdas
• Service layer: Updated lambdas in StudentService, CourseService, CertificateService
• Domain entities: Updated computed property lambdas (e.g., CurrentEnrollment)

Example

Before:

return await _dbSet
    .Include(s => s.Registrations)
        .ThenInclude(r => r.Course)
    .Where(s => s.IsActive)
    .FirstOrDefaultAsync(s => s.StudentId == studentId);

After:

return await _dbSet
    .Include(student => student.Registrations)
        .ThenInclude(registration => registration.Course)
    .Where(student => student.IsActive)
    .FirstOrDefaultAsync(student => student.StudentId == studentId);

8 files modified, zero functional changes.

Original prompt

Suggest more descriptive variable and function names

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