v2.0.0
Removed v1 plan payload aliases and enforced canonical key names.
Required migration
Replace `plan_code` with `key` in create and update payloads.
Update validation layer to reject unknown alias fields.
v1.9.0
Webhook signature algorithm changed from SHA1 to HMAC-SHA256.
Required migration
Recompute signatures using HMAC-SHA256 with shared secret.
Reject webhook payloads missing timestamp and signature headers.