docs: mark Phase 3 (MVP Finalization) as completed
Update project status to reflect completion of all MVP features:
Phase 3 achievements:
- ✅ Security hardening (CSRF, rate limiting, account lockout)
- ✅ All tests passing (223/223, 71% coverage)
- ✅ PWA features (manifest, service worker, iOS support)
- ✅ Production deployment preparation
- ✅ Operations scripts (backup, restore, health-check)
- ✅ Monitoring documentation
Status: 100% MVP complete, ready for production deployment
Next: Infrastructure setup (server, domain, SSL)
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