refactor: migrate worker functionality to outbox use case

[allisson]

Replace the worker package with a proper use case implementation in the outbox domain, following Clean Architecture principles and improving extensibility.

Changes:

• Create internal/outbox/usecase/outbox_usecase.go with OutboxUseCase implementation that handles event processing with configurable retry logic and transaction management
• Introduce EventProcessor interface for extensible event handling, allowing custom processors to be plugged in
• Add DefaultEventProcessor as the default implementation for basic event processing with logging
• Migrate all tests from worker package to outbox usecase with full coverage (78.8%)
• Update DI container (internal/app/di.go) to initialize and provide OutboxUseCase instead of EventWorker
• Update cmd/app/main.go runWorker function to use OutboxUseCase
• Remove internal/worker package entirely

Documentation updates:

• Update README.md project structure to show complete outbox module with domain, usecase, and repository layers
• Enhance README.md transactional outbox pattern section to explain use case-based approach and extensibility via EventProcessor
• Rewrite docs/architecture.md transactional outbox section with detailed explanations of the new architecture, including code examples for custom event processors
• Update docs/getting-started.md to clarify that worker command runs the outbox event processor
• Update docs/development.md to reflect new terminology

Benefits:

• Better architecture: Follows Clean Architecture with proper separation of concerns
• Improved extensibility: EventProcessor interface allows custom event handling without modifying core logic
• Enhanced maintainability: Outbox functionality now properly organized within the outbox domain
• Consistent with existing patterns: Matches the structure of other domains (user, etc.)
• Same functionality: Worker command continues to work identically