fix(proxy): override Vercel build command so it doesn't run turbo (#13448)

The proxy is intentionally outside the pnpm workspace and has no build
step (it ships as Vercel Functions compiled from api/). Vercel was
auto-detecting Turbo and defaulting the build command to `turbo run
build`, which fails with "Could not find task build in project" because
the isolated proxy package has no turbo.json or build task.

Add a vercel.json with an explicit no-op buildCommand so Vercel skips
turbo; the api/ functions are still detected and deployed.

Author: Bekacru

apps/proxy/vercel.json

@@ -0,0 +1,4 @@
+{
+  "$schema": "https://openapi.vercel.sh/vercel.json",
+  "buildCommand": "echo 'No build step required: the proxy is deployed as Vercel Functions from api/'"
+}