fix(billing): a returning customer's subscription reaches their team

The provider reuses a customer it already has under the same email and
leaves our team id off it, so the subscription arrived with no team,
was acknowledged, and changed nothing: a paid plan never applied. The
checkout now carries the team in its metadata, which the subscription
keeps, and the webhook reads it there when the customer has none. A
paid delivery that still matches no team is logged instead of dropped
in silence.

packages/core/src/billing/polar.test.ts

@@ -202,6 +202,39 @@ describe('Both signing schemes', () => {
 		expect(delivery.standing).toEqual({ plan: 'free', status: 'revoked' });
 	});
 
+	test('a customer who existed before checkout is found by the checkout metadata', async () => {
+		// The provider reuses a customer it already has under the same email and
+		// does not put our id on it, so the subscription arrives with a customer
+		// that has no external id. A paid subscription was dropped this way.
+		const { Webhook } = await import('standardwebhooks');
+		const secret = `whsec_${Buffer.from('c'.repeat(32)).toString('base64')}`;
+		configure({ POLAR_WEBHOOK_SECRET: secret });
+
+		const body = JSON.stringify({
+			type: 'subscription.active',
+			data: {
+				customer: { external_id: null },
+				metadata: { teamId: 'tem_returning' },
+				product_id: PAID
+			}
+		});
+		const id = 'msg_returning';
+		const timestamp = new Date();
+		const signature = new Webhook(secret).sign(id, timestamp, body);
+
+		const delivery = Polar.receive({
+			body,
+			headers: {
+				'webhook-id': id,
+				'webhook-timestamp': Math.floor(timestamp.getTime() / 1000).toString(),
+				'webhook-signature': signature
+			}
+		});
+
+		expect(delivery.teamId).toBe('tem_returning');
+		expect(delivery.standing).toEqual({ plan: 'paid', status: 'active' });
+	});
+
 	test('a tampered body under a valid-looking signature is refused', () => {
 		const secret = `whsec_${Buffer.from('a'.repeat(32)).toString('base64')}`;
 		configure({ POLAR_WEBHOOK_SECRET: secret });

packages/core/src/billing/polar.ts

@@ -187,6 +187,11 @@ export namespace Polar {
 			const created = await client().checkouts.create({
 				products: [productId],
 				externalCustomerId: input.teamId,
+				// Also on the checkout itself, which the provider copies onto the
+				// subscription. A customer who already existed under this email
+				// keeps the customer record they had, without our id on it, and
+				// this is then the only place the team survives.
+				metadata: { teamId: input.teamId },
 				customerEmail: input.email,
 				successUrl: input.successUrl
 			});
@@ -350,11 +355,15 @@ export namespace Polar {
 			const customer = data.customer as
 				| { externalId?: string | null; external_id?: string | null }
 				| undefined;
-			// `externalId` is the team id we put on the customer. A delivery
-			// without one is about a customer created some other way — by hand in
-			// their dashboard, most likely — and there is nothing here it can
-			// change.
-			const teamId = customer?.externalId ?? customer?.external_id ?? null;
+			// `externalId` is the team id we put on the customer. A customer who
+			// existed before checkout keeps the record they had, without it, so
+			// the team then comes from the checkout's metadata, which the
+			// subscription carries. A delivery with neither was not started here.
+			const metadata = data.metadata as { teamId?: unknown } | undefined;
+			const teamId =
+				customer?.externalId ??
+				customer?.external_id ??
+				(typeof metadata?.teamId === 'string' ? metadata.teamId : null);
 
 			const product = data.product as { id?: string } | undefined;
 			const productId =