IAV-1331/fix(credential): allow duplicate (credential_id, instance_name) for credential replication
Summary
Removes the @UniqueConstraint on (credential_id, instance_name) from the
credential_instances table to support credential replication within the same
N8N instance.
Context & Motivation
The customer-care N8N instance requires two separate CredentialInstance
records pointing to the same Credential under the same instance_name.
This replication pattern is intentional — both instances consume identical
credentials but are registered as distinct deployment slots.
The previous unique constraint blocked this use case at the database level,
causing a constraint violation when the second CredentialInstance was
persisted.
Impact
- No schema migration needed beyond dropping the existing unique index
credential_instances(credential_id, instance_name). - Uniqueness enforcement, if still required for other scenarios, must now be handled at the service/application layer.
- No functional regression expected for existing single-instance deployments.