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/src/domains/packs/schema.ts

@@ -61,7 +61,9 @@ export const responses = pgTable('responses', {
   id: uuid('id').primaryKey(),
   userId: uuid('user_id').notNull().references(() => profiles.id, { onDelete: 'cascade' }),
   questionId: uuid('question_id').notNull().references(() => questions.id, { onDelete: 'cascade' }),
-  coupleId: uuid('couple_id').notNull().references(() => couples.id, { onDelete: 'cascade' }),
+  // Nullable: an unpaired user's answer is banked as a sealed answer with no couple
+  // yet. It is claimed into the couple at pairing time; see the answers repository.
+  coupleId: uuid('couple_id').references(() => couples.id, { onDelete: 'cascade' }),
   answer: text('answer').notNull(),
   responseData: jsonb('response_data'),
   createdAt: timestamp('created_at', { withTimezone: true }).notNull().defaultNow(),