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/lib/coupleApi.ts

@@ -4,6 +4,8 @@ import { apiRequest } from "./apiClient";
 export interface CoupleStateResponse {
     couple: Couple | null;
     partner: Profile | null;
+    /** Count of this user's banked answers with no couple yet (couple_id IS NULL). */
+    sealed_count: number;
 }
 
 export interface CoupleMutationResponse {

apps/mobile/src/lib/inviteShareCopy.ts

@@ -0,0 +1,14 @@
+/**
+ * Builds the message shared when inviting a partner to pair. Once the inviter has
+ * sealed answers banked (answered questions solo, before a partner joined), the
+ * copy leads with that earned value instead of a plain invite, because it gives
+ * the recipient a concrete reason to join right away.
+ */
+export function buildInviteShareMessage(inviteCode: string, sealedCount: number): string {
+    const link = `https://sauci.app/join/${inviteCode} (or enter code ${inviteCode} in the app)`;
+    if (sealedCount > 0) {
+        const noun = sealedCount === 1 ? "question" : "questions";
+        return `Join me on Sauci! I have already answered ${sealedCount} ${noun} about us. Tap this link to unlock them: ${link}`;
+    }
+    return `Join me on Sauci! Tap this link to pair up instantly: ${link}`;
+}