License activation recovery: Allow instance_name in validate endpoint

Summary

Add support for recovering instance_id after network failures during license activation. Either make activate idempotent, or allow instance_name as an alternative to instance_id in the validate endpoint.

Problem

When calling POST /v1/licenses/activate, if the network drops before the response reaches the client, the instance_id is lost. Since /v1/licenses/validate requires instance_id, there's no way to recover.

I've tested and confirmed that calling activate again with the same key + instance_name creates a new instance (not idempotent), which increments activation_usage.

Proposed Solutions

1. Idempotent activation If activate is called with the same key + instance_name, return the existing instance instead of creating a new one. (This might be a breaking change)

2. Allow instance_name in validate (less invasive) Accept instance_name as an alternative to instance_id in the validate endpoint.

   POST /v1/licenses/validate { key, instance_name } 

   This would be non-breaking since it just adds a new optional parameter.

Reference

• Discussion:

  https://discord.com/channels/1262843562140241952/1467467311320666258/1467467311320666258

• LemonSqueezy has the same issue: https://lemonsqueezy.nolt.io/733

• Keygen.sh handles this with fingerprint scope: https://keygen.sh/docs/choosing-a-licensing-model/floating-licenses/

• Creem Activate API: https://docs.creem.io/api-reference/endpoint/activate-license

• Creem Validate API: https://docs.creem.io/api-reference/endpoint/validate-license