refactor(portal): extract shared partner types and data composables

Move partner domain types out of data/customers.ts into types/partner.ts so the fixture data exports can be removed later without breaking type imports. Add usePartnerTenants / usePartnerMrr composables wrapping the shared-key partner fetches.

apps/portal/composables/usePartnerTenants.ts

@@ -0,0 +1,21 @@
+// Shared fetch of the signed-in partner's customer tenants from
+// /api/partner/tenants (platform-api /me/partner/tenants). Several pages need
+// this list — dashboard, customers, and (later) billing — and they all key the
+// fetch as 'partner-tenants', so Nuxt dedupes to a single round-trip and shared
+// payload. Centralizing it here removes the divergent inline PartnerTenantDoc
+// copies that drifted between pages.
+//
+// Synchronous (non-async) so `useFetch` is invoked directly in the caller's
+// setup context — safe to call after another `await` in setup, unlike an async
+// wrapper. Call WITHOUT await:
+//   const { tenants, refresh } = usePartnerTenants()
+
+import type { PartnerTenantDoc } from '~/types/partner'
+
+export function usePartnerTenants() {
+  const { data: tenants, refresh, error, pending } = useFetch<PartnerTenantDoc[]>(
+    '/api/partner/tenants',
+    { key: 'partner-tenants', default: () => [] },
+  )
+  return { tenants, refresh, error, pending }
+}