feat(operator): partner management with attach/detach (O.6)

- Partners list with name/domain/status/customers/margin + Create modal
- Partner detail: contract card, contact card, customers table, attach modal,
  terminate (soft-delete) danger card
- Operator proxies for /partners + /partners/:slug/tenants
- platform-api: add partnerId Prop to Tenant schema. The field was being
  silently dropped by Mongoose because the schema didn't declare it.
- tenants.service: rewrite update() to build $set/$unset explicitly and cast
  partnerId via new Types.ObjectId(). Handles detach via $unset so the field
  vanishes from the doc cleanly.

apps/operator/server/api/partners/[slug]/index.get.ts

@@ -0,0 +1,6 @@
+import { platformApi } from '~~/server/utils/platform-api'
+
+export default defineEventHandler(async (event) => {
+  const slug = getRouterParam(event, 'slug')
+  return platformApi(event, `/partners/${slug}`)
+})