Add MySQL Spring Boot autoconfiguration (KOJAK-37)

[endrju19]

Summary

• Add MysqlStoreConfiguration to OutboxAutoConfiguration — zero-config MysqlOutboxStore + Liquibase migration when okapi-mysql is on the classpath
• Fix proxyBeanMethods = false on PostgresStoreConfiguration (Kotlin final classes + CGLIB proxy)
• Rename Postgres Liquibase bean to okapiPostgresLiquibase to avoid collision when both modules coexist

When both okapi-postgres and okapi-mysql are on the classpath, Postgres takes priority (declaration order + @ConditionalOnMissingBean). Users can override by defining their own @Bean OutboxStore.

Test plan

• MySQL E2E test: publish → claim → deliver → status update via Testcontainers MySQL 8.0
• Existing Postgres E2E test still passes (no regressions)
• Full test suite: 23/23 pass in okapi-spring-boot, all other modules green
• ktlint check passes