fix(keys): remove stale user update on delete

[rissrice2105-agent]

Summary

• remove the stale users table update from /api/keys/delete
• keep key deletion scoped to the authenticated user's user_public_keys row
• add regression coverage for successful deletion and delete failures

Why

The current endpoint tries to clear users.ml_kem_public_key with an eq('user_id', ...) filter after deleting user_public_keys. The migrations define users.id/auth_user_id, and user_public_keys.user_id references auth.users(id), so the extra users update is stale and can make key deletion report success incorrectly.

Validation

• corepack pnpm exec vitest run --config %TEMP%/qryptchat-vitest-no-react.config.mjs src/app/api/keys/delete/route.test.js
• corepack pnpm exec oxlint src/app/api/keys/delete/route.js src/app/api/keys/delete/route.test.js