add epoch notice

Author: maanex

content/2.api-v2/4.webhooks.md

@@ -39,6 +39,8 @@ FreeStuff signs every webhook request as per [Standard Webhooks Spec](https://gi
 - Make sure to take the raw http body (prior to parsing the JSON content) for the verification `payload`
 - Make sure to store the `msg_id` for a certain amount of time to prevent package duplication and replay attacks
 - Make sure to ignore requests with an old `Webhook-Timestamp` value
+  - The timestamp is stored as seconds since `2025-01-01`
+  - To calculate the current time multiply by `1000`, then add `1735689600000`
 - Make sure to use the payload body `timestamp` field as the contextual timestamp and use the header only for verification
 
 ::alert{type="info"}