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.

docs/OPERATOR-PLAN.md

@@ -415,13 +415,20 @@ forward as bearer to platform-api.
 - Nitro can throw `Could not load /app/server/api/...` if Vite picks
   up a new file mid-build. Container restart clears it.
 
-### O.6 · Partner management (real backend)
+### O.6 · Partner management (real backend) ✓
 
-- [ ] `pages/partners/index.vue` — list with name/domain/status/customers/MRR
-- [ ] `pages/partners/[slug].vue` — detail panel with customers list,
-      MRR breakdown, margin, contact info
-- [ ] "Create partner" modal — POST /partners
-- [ ] Attach / detach tenant to partner (PATCH on tenant.partnerId)
+- [x] `pages/partners/index.vue` — list with name/domain/status/customers/margin
+- [x] `pages/partners/[slug].vue` — detail panel with contract card, contact
+      card, customers table, attach modal, terminate danger card
+- [x] "Create partner" modal — POST /partners, navigates to detail on success
+- [x] Attach / detach tenant to partner (PATCH on tenant.partnerId, with $unset
+      for detach so the field disappears cleanly)
+- [x] `services/platform-api/src/schemas/tenant.schema.ts` — added the
+      `partnerId` Prop. It was missing, which is why early PATCH attempts
+      returned 200 but Mongoose silently dropped the field. Smoke-tested with
+      acme ⇄ nordicmsp and a throwaway temp-msp partner (created + terminated).
+- MRR aggregation deferred until Subscription gains real pricing (see
+  follow-ups). For now `customers` is just a count of attached tenants.
 
 ### O.7 · Visual-only screens (mock fixtures)