Backend - Implement exchange rate provider for Czech National Bank

[Nandomir]

Summary

Implement exchange rate provider that fetches and processes currency rates from the Czech National Bank (CNB) daily exchange rate service.

Changes

• Implemented CNB API client with retry policy and error handling
• Added data parser for CNB text format [1]
• Created exchange rate builder for currency conversions
• Added error handling with Result pattern
• Implemented DI and simple configuration
• Implemented logging
• Added unit test coverage

Testing

• All unit tests are green ✅
• Tested error scenarios and edge cases

Configuration

The provider can be configured via appsettings.json:

{
  "CnbApi": {
    "CnbDailyRatesUrl": "https://www.cnb.cz/en/financial-markets/foreign-exchange-market/central-bank-exchange-rate-fixing/central-bank-exchange-rate-fixing/daily.txt",
    "TimeoutSeconds": 30,
    "RetryCount": 3
  }
}