Add per-delivery prompts to Custom Webhooks

[laotoutou]

Summary

• accept an optional top-level prompt in authenticated Custom Webhook JSON and form deliveries
• keep the owner-configured Base prompt authoritative while separating caller instructions from untrusted event data
• label the form clearly and show a copyable curl example during setup
• update the architecture and generated OpenAPI contract

Testing

• full Node 24 test suite
• targeted Webhook ingress and prompt rendering tests
• typecheck
• lint
• OpenAPI generation and validation
• production web and server build

PostgreSQL-backed Webhook tests are present but skip locally because this Environment has no PostgreSQL server; CI should execute them with DATABASE_URL.