fix(security): resolve critical and high technical debt from audit

fix(audit): resolve remaining TypeScript any types, string enums, and Json validation

fix(audit): resolve 15 critical and major technical debt issues

feat(agentic-week2-4): 5 major features — AI content generator, handoff, tags, soft-delete, SSE

feat: backlog P0→P3 — toast system, payments, tenant isolation, feedback handler, i18n parity