Fix auth redirect to 0.0.0.0 and credentials sign-in error

- Add AUTH_URL env var to docker-compose.yml and .env/.env.example so
  NextAuth builds correct redirect URLs instead of falling back to the
  Docker bind address (0.0.0.0:3000)
- Normalize email to lowercase in authorize() to match how the register
  route stores emails, preventing case-sensitive lookup mismatches

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -14,6 +14,7 @@ services:
       - NEXT_PUBLIC_PREDICTIONS_ENABLED=true
       - API_KEY=${API_KEY}
       - AUTH_SECRET=${AUTH_SECRET}
+      - AUTH_URL=${AUTH_URL}
       - AUTH_GOOGLE_ID=${AUTH_GOOGLE_ID}
       - AUTH_GOOGLE_SECRET=${AUTH_GOOGLE_SECRET}
       - AUTH_TRUST_HOST=${AUTH_TRUST_HOST}