fix: throw on missing env vars in backend createServerSupabase()

From the PR description

Summary

• backend/src/lib/supabase.ts now throws Error("SUPABASE_URL and SUPABASE_SECRET_KEY must be set") when either env var is absent, matching the frontend fix from PR #73
• Removes dead getUserIdFromRequest export (no callers) which still used the old || "" pattern

Closes #97 Closes #114 Closes #117 Closes #124

Changes

• backend/src/lib/supabase.ts - replace || "" fallbacks with explicit guard; remove dead getUserIdFromRequest export
• backend/src/lib/__tests__/supabaseServer.test.ts - new test: throws on missing URL, missing key, both missing; succeeds when both set
• backend/vitest.config.ts - vitest configuration with include filter and isolate: true; "test" script added to package.json

Test plan

• Unit tests added and passing (4/4)
• Backend build passes