Make portal backend URL configurable

[suryabulusu]

Summary

• read reporting auth verification backend from mandatory PORTAL_BACKEND_URL
• pass PortalBackendUrl through SAM prod/staging templates
• wire deploy workflows to forward the GitHub secret into Lambda env

Required secrets

• Production: PORTAL_BACKEND_URL should point to production portal-backend
• Staging: PORTAL_BACKEND_URL should point to staging portal-backend

Verification

• parsed app/auth/init.py with Python AST
• checked prod/staging templates contain PORTAL_BACKEND_URL wiring