fix: add runtime derivation details for GITEA_INTERNAL_URL and GITEA_BACKUP_INTERNAL_URL in API documentation

contracts/gitea-api.md

@@ -7,6 +7,9 @@ All API calls used by the migration scripts. Every `gitea_api`, `gitea_backup_ap
 ## Gitea REST API v1
 
 Base URL: `${GITEA_INTERNAL_URL}/api/v1` (primary) or `${GITEA_BACKUP_INTERNAL_URL}/api/v1` (backup)
+Derived at runtime from container IPs:
+- `GITEA_INTERNAL_URL=http://${UNRAID_GITEA_IP}:3000`
+- `GITEA_BACKUP_INTERNAL_URL=http://${FEDORA_GITEA_IP}:3000`
 Auth: `Authorization: token ${GITEA_ADMIN_TOKEN}` (or `GITEA_BACKUP_ADMIN_TOKEN`)
 
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