Feat: outbox events

[misterquestions]

[Unreleased]

Added

• Transactional outbox implementation (entity, repository, relay with exponential backoff & retry)
• Event envelope (eventId, type, version, occurredAt, aggregateType, aggregateId, traceId, data)
• Micrometer metrics for outbox: orders.outbox.pending, orders.outbox.failed, orders.outbox.backlog.oldest_age_seconds, orders.outbox.relay.batch.duration, orders.outbox.publish.attempt{status}
• Health indicator (DEGRADED on failed rows) surfaced via /actuator/health
• Admin endpoints: POST /api/v1/outbox/requeue-failed, POST /api/v1/outbox/{id}/requeue
• Comprehensive test suite (relay success, retry -> success, forced failure -> FAILED, schema validation, optimistic locking, validation errors, requeue endpoint)
• JSON Schema validation tests for order.created.v1 & order.cancelled.v1
• Awaitility-based polling in async integration tests
• README enhancements (event envelope example, metrics, health, admin endpoints, CI badge)
• OpenAPI spec updates (outbox endpoints, quantity field normalization)
• Flyway migration V2__outbox.sql (if Flyway enabled) for outbox table

Changed

• Removed afterCommit direct Kafka publishing in favor of transactional outbox persistence
• Simplified EventPublisher (single-attempt publish)
• Refactored event payload classes to domain-only (moved type/version to envelope)
• Improved domain model with @Version and lifecycle timestamp callbacks

Fixed / Hardening

• Increased reliability & visibility for event publishing failures
• Added validation & optimistic locking test coverage

Security / Observability

• Structured logging for publish failures (attempt, status)
• Outbox backlog and failure counts exposed via metrics & health

Deprecated

• (None)

Removed

• Legacy afterCommit event publishing logic

Breaking Changes

• Event JSON shape changed (type/version now at envelope root; legacy consumers may need adaptation)