feat: implement tenant isolation with PostgreSQL RLS

- users + tenant_memberships tables (tenant-scoped)
- RLS policy on tenant_memberships using set_config('app.current_tenant_id', ...)
- withTenantContext() helper for transaction-scoped tenant context
- separate non-superuser app role (b2bcall_app): the default Docker postgres
  user is SUPERUSER and always bypasses RLS even with FORCE, so the app must
  never connect through the migration/owner role. Documented in
  docs/TENANT_ISOLATION.md.
- automated isolation test proving tenant A never sees tenant B's data

TODO.md

@@ -19,8 +19,14 @@
 - [x] Tabela `tenants` criada via migration (seção 29 do agente.md)
 
 ## PHASE 03 — Tenant Isolation
-- [ ] Tabela `tenants` + RLS
-- [ ] Tenant context em transação PostgreSQL
+- [x] Tabelas `users` + `tenant_memberships` (tenant-scoped)
+- [x] RLS (`ENABLE`/`FORCE ROW LEVEL SECURITY` + policy) em `tenant_memberships`
+- [x] Tenant context via `set_config('app.current_tenant_id', ..., true)` (transaction-local)
+- [x] Helper `withTenantContext()` em `packages/database`
+- [x] Role de banco separado para runtime (`b2bcall_app`, sem SUPERUSER/BYPASSRLS) —
+      achado crítico: o role padrão do Docker Postgres é SUPERUSER e SEMPRE ignora RLS,
+      até com FORCE. Ver `docs/TENANT_ISOLATION.md`.
+- [x] Teste automatizado de isolamento (`pnpm --filter @b2bcall/database run test:isolation`)
 
 ## PHASE 04 — Authentication / RBAC
 - [ ] Login (Argon2id), access/refresh tokens