feat(billing): implement REST API, database config, and tests for billing service

- **REST API:** Added `BillingController` with endpoints for managing participant accounts and transactions, including history retrieval.
- **Database Configuration:** Introduced `BillingDatabaseConfiguration` to initialize database schema using Exposed.
- **Testing:** Added integration tests for `TeilnehmerKontoService` using H2 in-memory database.

backend/services/billing/billing-service/src/test/kotlin/at/mocode/billing/service/TeilnehmerKontoServiceTest.kt

@@ -0,0 +1,64 @@
+@file:OptIn(ExperimentalUuidApi::class)
+
+package at.mocode.billing.service
+
+import at.mocode.billing.domain.model.BuchungsTyp
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+import org.springframework.beans.factory.annotation.Autowired
+import org.springframework.boot.test.context.SpringBootTest
+import org.springframework.test.context.ActiveProfiles
+import kotlin.uuid.ExperimentalUuidApi
+import kotlin.uuid.Uuid
+
+@SpringBootTest
+@ActiveProfiles("test")
+class TeilnehmerKontoServiceTest {
+
+    @Autowired
+    lateinit var service: TeilnehmerKontoService
+
+    @Test
+    fun `Konto erstellen und buchen`() {
+        val veranstaltungId = Uuid.random()
+        val personId = Uuid.random()
+        val personName = "Max Mustermann"
+
+        // 1. Konto erstellen
+        val konto = service.getOrCreateKonto(veranstaltungId, personId, personName)
+        assertNotNull(konto)
+        assertEquals(personName, konto.personName)
+        assertEquals(0L, konto.saldoCent)
+
+        // 2. Buchung durchführen
+        val updatedKonto = service.buche(
+            kontoId = konto.kontoId,
+            betragCent = 1500L,
+            typ = BuchungsTyp.NENNGEBUEHR,
+            zweck = "Nennung Bewerb 1"
+        )
+
+        assertEquals(1500L, updatedKonto.saldoCent)
+
+        // 3. Buchungshistorie prüfen
+        val buchungen = service.getBuchungsHistorie(konto.kontoId)
+        assertEquals(1, buchungen.size)
+        assertEquals(1500L, buchungen[0].betragCent)
+        assertEquals("Nennung Bewerb 1", buchungen[0].verwendungszweck)
+    }
+
+    @Test
+    fun `Mehrere Buchungen summieren sich korrekt`() {
+        val vId = Uuid.random()
+        val pId = Uuid.random()
+        val konto = service.getOrCreateKonto(vId, pId, "Susi Sorglos")
+
+        service.buche(konto.kontoId, 2000L, BuchungsTyp.STARTGEBUEHR, "Startgeld")
+        val finalKonto = service.buche(konto.kontoId, -500L, BuchungsTyp.STORNIERUNG, "Storno")
+
+        assertEquals(1500L, finalKonto.saldoCent)
+
+        val historian = service.getBuchungsHistorie(konto.kontoId)
+        assertEquals(2, historian.size)
+    }
+}

backend/services/billing/billing-service/src/test/resources/application-test.yml

@@ -0,0 +1,9 @@
+spring:
+  datasource:
+    url: jdbc:h2:mem:billing_test;DB_CLOSE_DELAY=-1
+    driver-class-name: org.h2.Driver
+    username: sa
+    password: ""
+  h2:
+    console:
+      enabled: true