feat: Claude Sonnet 4.6 provider + Stripe billing automation

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

chore: stabilization, audit fixes and project synchronization

chore: finalize Sprint P2 & P3 optimizations, baseline prisma migrations, and update technical audit docs

feat: B2B SaaS Multi-tenant architecture & Tech Debt Resolution