Fix auth user profile sync

[cursor]

Bug and impact

Successful Supabase signup/login returned valid auth tokens but did not create or repair the corresponding application users row. New or orphaned authenticated users then hit USER_NOT_FOUND on /users/me and foreign-key failures on profile/preferences/meal-log flows.

Root cause

AuthService only interacted with Supabase Auth. The Prisma users table is the parent for all user-scoped app data, but no production auth path inserted or backfilled that row.

Fix

• Add a small syncUserRecord helper in AuthService that upserts the Prisma user row from the Supabase auth user.
• Run the sync on signup, login, and refresh before returning auth success.
• Preserve signup display names and backfill from Supabase metadata when repairing via login.
• Add focused auth endpoint tests for signup creation and login repair.

Validation

• pnpm install --frozen-lockfile
• pnpm --filter @snacktrack/shared-types build
• DATABASE_URL=postgresql://localhost:5432/snacktrack_test DIRECT_URL=postgresql://localhost:5432/snacktrack_test pnpm --filter @snacktrack/backend db:generate
• pnpm --filter @snacktrack/backend test -- auth.test.ts ✅
• DATABASE_URL=postgresql://localhost:5432/snacktrack_test DIRECT_URL=postgresql://localhost:5432/snacktrack_test pnpm --filter @snacktrack/backend build ✅
• pnpm --filter @snacktrack/backend test ⚠️ one pre-existing-looking failure remains: tests/health.test.ts expects 200 for all services healthy, but the health route also checks ML service health and that test does not mock mlService.isHealthy(), so it returns 503.