[codex] Remove unused Supabase request auth helper

From the PR description

Summary

Removes the unused getUserIdFromRequest helper from backend/src/lib/supabase.ts.

Why

The helper is not referenced anywhere in the repository. Authenticated backend routes use the existing Express middleware flow instead, so keeping this helper adds dead code and an extra request-style auth path that is no longer used.

Changes

• Deleted the unused request JWT helper from backend/src/lib/supabase.ts
• Left createServerSupabase unchanged

Testing

• rg "getUserIdFromRequest" . returns no matches
• git diff --check passes

Note: I could not run npm run build --prefix backend in this local shell because the checkout does not have dependencies installed.