Fix/name collision on import

[gallayl]

🎯 Summary

Hardens entity-creation endpoints against client-supplied id collisions:

• POST /services, /stacks, /github-repositories, /prerequisites now reject duplicate id/name with 409 Conflict and a descriptive message instead of silently overwriting state or surfacing a generic store error.
• POST /stacks/import gains an assertNoImportConflicts pre-flight that collects every colliding stack.name, service.id, repository.id, and prerequisite.id and returns a single 409. This prevents the existing rollback path from being entered for avoidable collisions (which would have deleted the pre-existing colliding records).
• Multi-step create flows (/services, /stacks, /prerequisites) roll back the records they have already inserted when a later step fails, so a failed create no longer leaves orphan rows.

🧪 Tests

• Duplicate-id 409 + check-result rollback specs for POST /prerequisites.
• Duplicate stack.name and cross-stack duplicate service.id specs for POST /stacks/import.