refactor: consolidate Liquibase migrations into a single changeset per database

[endrju19]

Summary

• Collapse okapi's incremental Liquibase changelogs into one 001__create_okapi_outbox_table.sql per database — PostgreSQL 4 changesets → 1, MySQL 3 → 1. Each changelog.xml now includes only the consolidated file; the per-step migration files are removed.
• Net schema for a new installation is unchanged (same table, columns, both indexes); new users get the full schema from a single changeset instead of replaying the change history.
• CHANGELOG documents the consolidation and that the outbox:001 checksum changes — existing installations must start on a fresh okapi schema, or clear okapi's rows from okapi_databasechangelog, before upgrading.

Test Plan

• LiquibaseE2ETest (PostgreSQL + MySQL Spring Boot autoconfig, real Liquibase on Testcontainers) — 8/8 passing
• Concurrency claim/purge integration tests (exercise the consolidated schema + indexes via real INSERT/claim) — 4/4 passing
• Full ./gradlew ktlintCheck clean