fix: adding a found subscription sends its membership id (adopt 400 invalid_body)

membershipReconcile.adopt passed { membership_id } as request options, so
no body was sent. useApi now refuses any option other than body, params or
signal, so a payload in the wrong place fails loudly in the browser instead
of as a 400 from the server.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

frontend/src/stores/membershipReconcile.js

@@ -28,7 +28,7 @@ export const useMembershipReconcileStore = defineStore('membershipReconcile', ()
   async function adopt (membershipId) {
     try {
       const res = await api.post('/api/sources/reconciliation/adopt', {
-        membership_id: membershipId
+        body: { membership_id: membershipId }
       })
       toast({
         text: res.already_tracked