Improve variable and function name descriptiveness across backend and frontend

[Copilot]

Generic and abbreviated names (result, e, el, submitBtn) throughout controllers, middleware, and JS event handlers reduce readability and make intent unclear at a glance.

Backend (C#)

• Controllers — replaced generic result with type-intent names:
  • pagedStudents, pagedCourses, pagedRegistrations for paged query returns
  • isStudentDeleted, isCourseDeleted, isRegistrationCancelled for boolean returns
• Repository.cs — result → addedEntityEntry (EF EntityEntry<T>)
• AdminAccessChecker.cs — result → adminValidationResult
• ExceptionHandlingMiddleware.cs — response → httpResponse, errorResponse → apiErrorResponse, jsonOptions → camelCaseJsonOptions

Frontend (JavaScript)

• submitBtn → submitButton
• Single-letter event params e → context-specific names: inputEvent, keyboardEvent, clickEvent, submitEvent
• el in forEach callbacks → errorElement / inputElement

// Before
document.addEventListener('keydown', function(e) {
    if (e.key === 'Escape') { ... }
});
errorElements.forEach(el => el.textContent = '');

// After
document.addEventListener('keydown', function(keyboardEvent) {
    if (keyboardEvent.key === 'Escape') { ... }
});
errorElements.forEach(errorElement => errorElement.textContent = '');

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