In-Memory Expense Repository

Implement in Solution: - record Expense(String id, String category, int cents) - static class ExpenseRepository with: - Expense save(Expense e) (upsert by id) - Optional<Expense> findById(String id) - List<Expense> findByCategory(String category) (insertion order) - boolean deleteById(String id) - List<Expense> findAll() (insertion order) - int totalCents() - static class DuplicateExpenseException extends RuntimeException. save acts as a category-guarded upsert: same id + same category replaces the entry (updates allowed); same id + DIFFERENT category throws DuplicateExpenseException (an id is bound to one category).

Difficulty: intermediate