module-12. add outbox

[s-aleshin]

feat(outbox): implement transactional domain event publishing. outbox pattern

• Add OutboxMessage model, migrations, factories, and processor tests

• Implement OutboxDomainEventPublisher and integration event processing job

• Move domain event publishing under database transaction

• Add DI configuration for OutboxDomainEventPublisher

• Support publishing integration events to multiple Kafka topics

• Enhance outbox payload serialization and message structure

• Switch PostgreSQL driver from asyncpg to psycopg

• Remove asyncpg and granian dependencies

• Add unit tests for outbox and integration event publishers

• Update UUID types to SQLAlchemy Uuid to avoid json serialization issues

• Refactor domain events to use explicit serializable payloads and remove aggregate source references

NOTES
@kvetchinkin after our discussion of the events in the course chat I’ve made some refactoring which brings some noise to the PR. To simplify the review process, I listed the main changes that affect the module task below:

1. OutboxMessage ORM model
2. OutboxDomainEventPublisher - collects domain events from aggregates and stores them in the DB as OutboxMessage.
3. OutboxMessageProcessor - callable class used by the background job to process outbox messages and publish them to Kafka.
4. Background Job for procession outbox messages
5. The background job connected to the scheduler
6. Integration Events Publisher - tried to implement implemented as a more generic publisher, not tied to one specific aggregate.