test: add unit tests for PaymentsService and MaintenanceWorker #424

Problem

PaymentsService and MaintenanceWorker contain critical business logic (invoice lifecycle, fee calculations, scheduled invoice polling) but have very low or zero test coverage:

• src/services/payments-service.ts — 8% statement coverage
• src/app/maintenance-worker.ts — 0% coverage

Proposed Solution

Add unit tests for both modules using existing patterns (Mocha + Chai + Sinon):

payments-service.spec.ts (~29 tests):

• getPendingInvoices() — delegation and error propagation
• getInvoiceFromPaymentsProcessor() — string vs Invoice input handling, verifyURL logic
• createInvoice() — transaction lifecycle (begin/commit/rollback), field mapping
• updateInvoice() / updateInvoiceStatus() — delegation to repository
• confirmInvoice() — validation guards, SATS/BTC/MSATS unit conversions, admission fee schedule filtering with pubkey whitelists, user admission on threshold met, transaction rollback on error
• sendInvoiceUpdateNotification() — event construction, MSATS->SATS conversion, error logging in pipeline

maintenance-worker.spec.ts (~17 tests):

• Signal handler registration
• Interval scheduling
• Skips processing when payments disabled
• Invoice polling loop with per-invoice error resilience
• Status change detection (PENDING -> COMPLETED)
• amountRequested fallback for amountPaid via mergeDeepLeft
• close(), onError(), onExit() behavior

Impact

Raises overall unit test coverage from ~50% to ~55% statements. Both files reach 100% statement/function coverage.