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/mobile/src/__tests__/features/swipe/useSwipeScreen.test.ts

@@ -2,6 +2,7 @@ import { act, renderHook, waitFor } from "@testing-library/react-native";
 
 import { useSwipeScreen } from "@/features/swipe/hooks/useSwipeScreen";
 import { ApiError, apiClient } from "@/lib/apiClient";
+import { useAuthStore } from "@/store";
 import * as swipeService from "@/features/swipe/services/swipeService";
 import { getSkippedQuestionIds } from "@/lib/skippedQuestions";
 
@@ -15,12 +16,13 @@ jest.mock("expo-router", () => ({
 // Stable identities: the hook's effects key off these, so fresh objects per render
 // would retrigger the fetch effect forever and the screen would never settle.
 jest.mock("@/store", () => {
-    const auth = { user: { id: "user-1" }, partner: { id: "user-2" }, couple: { id: "couple-1", couple_id: "couple-1" } };
+    const auth = { user: { id: "user-1" }, partner: { id: "user-2" }, couple: { id: "couple-1", couple_id: "couple-1" }, sealedCount: 0 };
     const packs = { enabledPackIds: ["pack-1"], ensureEnabledPacksLoaded: async () => undefined, packs: [] };
     const packState = { ...packs, invalidatePacks: jest.fn() };
     const responses = { invalidateResponses: jest.fn() };
     const useAuthStore = () => auth;
     useAuthStore.getState = () => auth.user;
+    useAuthStore.setState = jest.fn();
     const usePacksStore = () => packs;
     usePacksStore.getState = () => packState;
     const useResponsesStore = () => responses;
@@ -146,6 +148,19 @@ describe("useSwipeScreen daily limit", () => {
         expect(result.current.dailyLimitInfo).toMatchObject({ responses_today: 5, remaining: 5 });
     });
 
+    it("updates the sealed answer count optimistically when the server reports one", async () => {
+        // A solo answer has no partner to match against, so sealed_count is the
+        // only feedback the server gives back; the banner reads it from auth state.
+        const { result } = await renderReady();
+
+        jest.spyOn(apiClient, "post").mockResolvedValueOnce({ match: null, sealed_count: 3 });
+        await act(async () => {
+            await result.current.handleAnswer("q1", "yes");
+        });
+
+        expect(useAuthStore.setState).toHaveBeenCalledWith({ sealedCount: 3 });
+    });
+
     it("refreshes the shared streak once the answer lands", async () => {
         const { result } = await renderReady();
         mockFetchStreak.mockClear();