Feature/domain models

[soraiayugulis]

Summary

Implements all core domain entities and value objects for the Credit Card Authorizer API following TDD principles, as specified in Phase 1 of the implementation plan.

Changes

Entities

• Card — Card entity with 16-digit number validation, holder name validation, category balance management, and ACTIVE/INACTIVE status
• Merchant — Merchant entity with name validation, single category assignment, and ACTIVE/INACTIVE status
• Transaction — Transaction entity tracking requested category, source category (for fallback), APPROVED/DENIED status, authorization codes, and rejection reasons

Value Objects

• Balance — Immutable value object with add/subtract operations, negative amount rejection, and timestamp tracking
• AuthorizationResult — Immutable result object with factory methods approved() and denied(), carrying authorization code, rejection reason, and source category

Enums

• Category — FOOD, MEAL, TRANSPORTATION, CULTURE, PHARMACY, GENERAL with MEAL → FOOD → GENERAL fallback chain
• CardStatus — ACTIVE, INACTIVE
• MerchantStatus — ACTIVE, INACTIVE
• TransactionStatus — APPROVED, DENIED

Tests

• 30 unit tests — all passing
• Covers creation, validation, immutability, fallback chain, and edge cases

Spec Coverage

Fact/Rule	Status
Fact 1: Card Registration	✅ Card entity with zero-balance init
Fact 8: Merchant Classification	✅ Merchant with single category
Rule 1: Card Number Validation	✅ 16-digit numeric validation
Rule 3: Category Fallback	✅ MEAL → FOOD → GENERAL chain
Rule 4: Merchant Category Mapping	✅ Immutable category assignment