fix(couples): protect joined invites from cancellation

apps/api/src/domains/couples/repository.ts

@@ -33,6 +33,7 @@ export interface CoupleRepository {
   getState(userId: string): Promise<CoupleStateResponse>;
   create(userId: string, coupleId: string, inviteCode: string): Promise<Couple>;
   join(userId: string, inviteCode: string): Promise<Couple>;
+  cancelInvite(userId: string): Promise<void>;
   cancel(userId: string): Promise<void>;
   close(): Promise<void>;
 }
@@ -268,6 +269,42 @@ export class PostgresCoupleRepository implements CoupleRepository {
     });
   }
 
+  async cancelInvite(userId: string): Promise<void> {
+    await transaction(this.pool, async (client) => {
+      // Lock in the same order as join(): the caller's profile first, then the
+      // couple. A concurrent join therefore either completes first and makes
+      // this a safe no-op error, or observes that the invite no longer exists.
+      const profile = await profileForUpdate(client, userId);
+      if (!profile.couple_id) {
+        throw new CoupleError('not_paired', 'You do not have an invite to cancel', 400);
+      }
+
+      const coupleResult = await client.query<{ id: string }>(
+        'select id from couples where id = $1 for update',
+        [profile.couple_id],
+      );
+      if (!coupleResult.rows[0]) {
+        throw new CoupleError('not_paired', 'You do not have an invite to cancel', 400);
+      }
+
+      const members = await client.query<{ id: string }>(
+        'select id from profiles where couple_id = $1 for update',
+        [profile.couple_id],
+      );
+      if (members.rows.length > 1) {
+        throw new CoupleError('invite_already_joined', 'This invite has already been joined', 409);
+      }
+      if (members.rows.length !== 1 || members.rows[0]?.id !== userId) {
+        throw new CoupleError('not_paired', 'You do not have an invite to cancel', 400);
+      }
+
+      // At this point the couple has precisely the authenticated creator as a
+      // member. Removing it cannot delete paired data, and the foreign key
+      // clears the creator's couple_id while leaving sealed answers intact.
+      await client.query('delete from couples where id = $1', [profile.couple_id]);
+    });
+  }
+
   async close(): Promise<void> {
     await closeResolvedPool(this.pool, this.ownsPool);
   }

apps/api/src/domains/couples/routes.ts

@@ -50,4 +50,9 @@ export function registerCoupleRoutes(app: AuthenticatedApp, service: CoupleServi
     const result = await route(() => service.cancel(c.get('identity').id));
     return result.ok ? c.json(result.value) : c.json(result.body, result.status);
   });
+
+  app.delete('/v1/couple/invite', async (c) => {
+    const result = await route(() => service.cancelInvite(c.get('identity').id));
+    return result.ok ? c.json(result.value) : c.json(result.body, result.status);
+  });
 }

apps/api/src/domains/couples/service.ts

@@ -41,4 +41,9 @@ export class CoupleService {
     await this.repository.cancel(userId);
     return { success: true, couple_id: null };
   }
+
+  async cancelInvite(userId: string): Promise<CoupleMutationResponse> {
+    await this.repository.cancelInvite(userId);
+    return { success: true, couple_id: null };
+  }
 }

apps/api/src/domains/couples/types.ts

@@ -19,6 +19,7 @@ export type CoupleErrorCode =
   | 'invalid_invite_code'
   | 'couple_full'
   | 'not_paired'
+  | 'invite_already_joined'
   | 'invite_code_collision';
 
 export class CoupleError extends Error {