Feature/authorization service

[soraiayugulis]

Summary

Implements the core authorization logic for the Credit Card Authorizer API, including validators, balance checking with MEAL fallback chain, daily transaction limits, and authorization code generation — all following TDD.

Changes

Validators

• CardNumberValidator — validates 16-digit numeric format
• TransactionAmountValidator — validates amount range (0.01–10000.00) with specific error codes (INVALID_AMOUNT, AMOUNT_EXCEEDED)
• ValidationResult — value object for validation outcomes

Authorization Components

• CategoryBalanceChecker — checks balance with MEAL → FOOD → GENERAL fallback chain; non-MEAL categories denied immediately on insufficient balance
• DailyTransactionLimitChecker — enforces 2000.00 daily limit per card, counting only today's APPROVED transactions
• AuthorizationCodeGenerator — generates unique 12-char alphanumeric codes in XXXX-XXXX-XXXX format using SecureRandom

Authorization Service

• AuthorizationService — orchestrates the full authorization flow:
  1. Card active check → CARD_INACTIVE
  2. Amount validation → INVALID_AMOUNT / AMOUNT_EXCEEDED
  3. Daily limit check → DAILY_LIMIT_EXCEEDED
  4. Balance check with fallback → INSUFFICIENT_BALANCE
  5. Balance deduction + code generation on success

Tests

• 65 unit tests — all passing
• Covers: valid/invalid card numbers, amount boundaries, all fallback scenarios, daily limit accumulation, code uniqueness, full authorization flow

Spec Coverage

Fact/Rule	Status
Fact 3: Successful Authorization	✅
Fact 4: Insufficient Balance	✅
Fact 5: Invalid/Inactive Card	✅
Fact 6: MEAL Fallback Chain	✅
Fact 7: Direct Category Authorization	✅
Rule 2: Amount Validation	✅
Rule 3: Balance Check with Fallback	✅
Rule 5: Authorization Code	✅
Rule 6: Concurrent Handling	✅ (atomic balance ops)
Rule 8: Daily Transaction Limit	✅