Add integration tests for migration categories and execution

- Implemented MigrationCategoryTests to validate migration categorization for startup, release, seed, and data migrations.
- Added tests for edge cases, including null, empty, and whitespace migration names.
- Created StartupMigrationHostTests to verify the behavior of the migration host with real PostgreSQL instances using Testcontainers.
- Included tests for migration execution, schema creation, and handling of pending release migrations.
- Added SQL migration files for testing: creating a test table, adding a column, a release migration, and seeding data.

docs/adr/index.md

@@ -34,8 +34,15 @@ Small, module-local refactors that do not modify public behaviour can live in co
 - [ ] Consequences call out migration or rollback steps.
 - [ ] Announcement posted to Docs Guild updates (or sprint log).
 
+## ADR Index
+
+| ADR | Title | Status | Date |
+|-----|-------|--------|------|
+| [0001](./0001-postgresql-for-control-plane.md) | PostgreSQL for Control-Plane Storage | Accepted | 2025-12-04 |
+
 ## Related resources
 - [Docs Guild Task Board](../TASKS.md)
 - [High-Level Architecture Overview](../07_HIGH_LEVEL_ARCHITECTURE.md)
+- [Database Documentation](../db/README.md)
 - [Coding Standards](../18_CODING_STANDARDS.md)
 - [Release Engineering Playbook](../13_RELEASE_ENGINEERING_PLAYBOOK.md)