feat(pairing): solo sealed answers that unlock on pairing

Unpaired users could only reach a waiting-room dead end and could not answer
questions until a partner joined. Responses.couple_id is now nullable so a
solo user answers immediately and banks "sealed answers" (couple_id IS NULL).
Pairing claims both members' sealed answers and computes matches for every
question both have now answered. Invite copy and the unpaired push nudge
escalate with the sealed answer count.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/api/test/couples.routes.test.ts

@@ -14,7 +14,7 @@ const identity: AuthIdentity = {
 
 function setup() {
   const repository: CoupleRepository = {
-    getState: vi.fn(async () => ({ couple: null, partner: null })),
+    getState: vi.fn(async () => ({ couple: null, partner: null, sealed_count: 0 })),
     create: vi.fn(async (_userId, coupleId, inviteCode) => ({
       id: coupleId,
       invite_code: inviteCode,
@@ -43,7 +43,7 @@ describe('couple routes', () => {
     const response = await app.request('/v1/couple');
 
     expect(response.status).toBe(200);
-    expect(await response.json()).toEqual({ couple: null, partner: null });
+    expect(await response.json()).toEqual({ couple: null, partner: null, sealed_count: 0 });
     expect(repository.getState).toHaveBeenCalledWith(identity.id);
   });