Log missing config warnings only once

[TaprootFreak]

Summary

Graceful degradation for services when external dependencies are unavailable in local development mode.

Commits (6)

1. Log missing config warnings only once - Initial warning-once pattern
2. Skip services gracefully when dependencies are unavailable - Add availability checks
3. Add missing null checks for graceful degradation - Handle null clients
4. Make warning-once pattern consistent across all services - Consistent Set
5. Add readonly to syncWarningsLogged Set for consistency - Code style fix
6. Add missing blank line after early return for consistency - Formatting fix

Pattern Overview

Pattern	Use Case	Services
private unavailableWarningLogged = false	Single dependency	BitcoinStrategy, CheckoutObserver, FiatPayInSyncService, BankTxService
private readonly ...Set<Blockchain>()	Multi-blockchain	PaymentBalanceService, LogJobService, TransactionHelper
private readonly ...Set<string>()	Multi-exchange	ExchangeTxService

Changes by File (10 files, +79/-10 lines)

File	Change
checkout.service.ts	Add isAvailable()
exchange-tx.service.ts	Error → Warning-once
checkout.observer.ts	Add availability check
payment.observer.ts	Fix null pointer
payment-balance.service.ts	Add null checks + warning-once
bank-tx.service.ts	Add warning-once
fiat-payin-sync.service.ts	Add availability check
log-job.service.ts	Add null check + warning-once
bitcoin.strategy.ts	Add warning-once
transaction-helper.ts	Add null check + warning-once

Quality Checklist

• All Set<> fields use readonly
• Consistent naming conventions
• Consistent message format
• Consistent blank lines after early returns
• TypeScript compiles without errors
• All tests pass (48/48)
• Manual verification: warnings appear only once
• Manual verification: no null pointer errors