chore: add make signing-key helper

Convenience target to generate a secp256k1 signing key in the hex
format expected by the services.

Eurosky fork change on top of upstream.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Spotchi

Makefile

@@ -71,3 +71,7 @@ nvm-setup: ## Use NVM to install and activate node+pnpm
 	nvm use
 	corepack enable
 	corepack install
+
+.PHONY: signing-key
+signing-key:
+	@openssl ecparam --name secp256k1 --genkey --noout --outform DER | tail --bytes=+8 | head --bytes=32 | xxd --plain --cols 32