Fix production referral service wiring

Bind the referral route interface to its service and extend the module regression test to cover every route-facing operations boundary.

src/test/kotlin/com/osglab/account/ApplicationTest.kt

@@ -5,6 +5,8 @@ import com.osglab.account.features.credits.repositories.BillingTransactionRunner
 import com.osglab.account.features.credits.repositories.BillingUnitOfWork
 import com.osglab.account.features.credits.services.CreditOperations
 import com.osglab.account.features.credits.services.CreditService
+import com.osglab.account.features.referrals.services.ReferralOperations
+import com.osglab.account.features.referrals.services.ReferralService
 import io.kotest.matchers.shouldBe
 import io.ktor.client.request.get
 import io.ktor.http.HttpStatusCode
@@ -27,7 +29,7 @@ class ApplicationTest {
     }
 
     @Test
-    fun `application module exposes credit operations through its interface`() {
+    fun `application module exposes route services through their interfaces`() {
         val transactionRunner = object : BillingTransactionRunner {
             override suspend fun <T> inTransaction(block: (BillingUnitOfWork) -> T): T =
                 error("Transaction runner is not used while resolving dependencies")
@@ -41,6 +43,8 @@ class ApplicationTest {
         try {
             application.koin.get<CreditOperations>() shouldBe
                 application.koin.get<CreditService>()
+            application.koin.get<ReferralOperations>() shouldBe
+                application.koin.get<ReferralService>()
         } finally {
             application.close()
         }