Tenant isolation (RLS) + AI cost-control stubs + docs

[haroondev999]

This PR adds several productionization changes:

• Tenant isolation: AsyncLocalStorage tenant context, Nest TenantInterceptor, Prisma middleware to scope queries by organization_id, and RLS policy SQL (backend/deploy/rls_policies.sql).
• AI cost-control: starter Nest module, service, and middleware to enforce per-organization budgets.
• Docs: Claude.production.md (production plan) and backend/docs/tenant_isolation.md.

Notes:

• These are initial stubs and migration helpers. Please review and run migrations/test in staging before promoting to production.
• CI will surface type errors; I'm happy to iterate on follow-up commits.

Co-authored-by: automated-agent devnull@example.com