fix(webhooks): reject blank webhook delete ids

[aiirvizionz]

Summary

• Reject blank webhook ids in the DELETE route before building the Supabase delete query.
• Trim the route param before matching the webhook id so whitespace-only ids cannot reach .eq('id', ...).
• Add a focused regression test that asserts blank ids return 400 and do not call delete.

Validation

• node --check src/app/api/webhooks/[id]/route.js
• node --check src/app/api/webhooks/[id]/route.test.js
• git diff --check

I also attempted the focused Vitest file, but this checkout fails before test collection because @vitejs/plugin-react imports Vite ./internal, which is not exported by the installed Vite package in this Windows/Node environment. The regression test is included in the PR for maintainers' normal CI/runtime.