chore: stabilization, audit fixes and project synchronization

feat: implement Phase 4 (Quota management, centralized logging, and env sync)

fix: remove hardcoded WhatsApp fallback and use environment variables

fix: update production WhatsApp number in web app

feat: admin auth login, real WhatsApp Cloud API, Cloudflare R2 storage

fix(security): audit fixes — Stripe webhook scope, fail-fast secrets, prisma singleton, worker auth, Zod validation, WA number env var