fix(customer-auth): make organization_id optional at registration

Requiring organization_id broke the current app, which does not send it.
When it is left out and the database has exactly one organization, the
customer now joins that one, so the app works unchanged. A sent
organization_id must still exist, and with several organizations and none
sent registration is refused with a clear message.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/processor/customer_auth_processor.go

@@ -143,19 +143,12 @@ func (p *customerAuthProcessor) StartRegistration(ctx context.Context, req *cont
 		return nil, fmt.Errorf("phone number already registered")
 	}
 
-	// The customer joins the organization the app is built for. Check it exists now,
-	// before an OTP is sent, rather than failing on a foreign key at the last step.
-	organizationID, err := uuid.Parse(strings.TrimSpace(req.OrganizationID))
-	if err != nil {
-		return nil, fmt.Errorf("organization_id must be a valid UUID")
-	}
-	orgExists, err := p.customerAuthRepo.OrganizationExists(ctx, organizationID)
+	// Resolve the organization before an OTP is sent, rather than failing on a foreign
+	// key at the last step.
+	organizationID, err := p.registrationOrganization(ctx, req.OrganizationID)
 	if err != nil {
 		return nil, err
 	}
-	if !orgExists {
-		return nil, fmt.Errorf("organization not found")
-	}
 
 	// Generate registration token and create OTP session
 	registrationToken := uuid.New().String()
@@ -458,3 +451,37 @@ func (p *customerAuthProcessor) ResendOtp(ctx context.Context, req *contract.Res
 }
 
 // Helper functions - OTP generation is now handled by OtpProcessor
+
+// registrationOrganization is the organization a new customer joins: the one the app
+// sent, which must exist, or, when the app sent none, the only organization there is.
+// With several organizations and none sent there is no way to choose, so it refuses.
+func (p *customerAuthProcessor) registrationOrganization(ctx context.Context, requested string) (uuid.UUID, error) {
+	requested = strings.TrimSpace(requested)
+	if requested != "" {
+		id, err := uuid.Parse(requested)
+		if err != nil {
+			return uuid.Nil, fmt.Errorf("organization_id must be a valid UUID")
+		}
+		exists, err := p.customerAuthRepo.OrganizationExists(ctx, id)
+		if err != nil {
+			return uuid.Nil, err
+		}
+		if !exists {
+			return uuid.Nil, fmt.Errorf("organization not found")
+		}
+		return id, nil
+	}
+
+	ids, err := p.customerAuthRepo.OrganizationIDs(ctx, 2)
+	if err != nil {
+		return uuid.Nil, err
+	}
+	switch len(ids) {
+	case 1:
+		return ids[0], nil
+	case 0:
+		return uuid.Nil, fmt.Errorf("no organization exists to register customers into")
+	default:
+		return uuid.Nil, fmt.Errorf("organization_id is required: there is more than one organization")
+	}
+}