docs(notification): add module documentation for invoice paid email feature
- Add notification/README.md with knowledge map and module connections
- Add docs/overview.md covering clean architecture layers, port design rationale (IInvoiceStoragePort kept separate from accounting to ease future microservice extraction), and InputStream resource management
- Add docs/usecase.md documenting SendInvoice orchestration, command field mapping from InvoiceDocumentPaidIntegrationEvent, and error propagation strategy for Kafka retry
- Add docs/events.md with full InvoiceDocumentPaidIntegrationEvent payload table and consumer group assignment
- Add docs/invoice_paid_email_workflow.md with Mermaid sequence diagram, Thymeleaf variable reference, S3 URL parsing logic, and error matrix
- Register notification-group in KAFKA_ARCHITECTURE.md consumer group table and billing.document.paid in the event-to-topic mapping
- Add section 4 "Customer Notifications" to COMPLETED_FEATURES.md
- Fix JavaMailEmailAdapter log line wrapping (IDE formatting only)
Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com