- **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.
- **Billing Domain:**
- Added Kotlin Multiplatform project with domain models (`TeilnehmerKonto`, `Buchung`, `BuchungsTyp`) to represent billing entities.
- Defined serialization strategies using `InstantSerializer`.
- **Service Implementation:**
- Introduced `BillingServiceApplication` as the main entry point for the billing service.
- Developed `TeilnehmerKontoService` for account management and transactions.
- **Persistence Layer:**
- Implemented Exposed repositories (`ExposedTeilnehmerKontoRepository`, `ExposedBillingRepositories`) for database interaction.
- Added table definitions (`TeilnehmerKontoTable`, `BuchungTable`) with indexes for efficient querying.
- **Build Configuration:**
- Setup Gradle build files for billing domain and service modules with dependencies for Kotlin, Serialization, Spring Boot, and Exposed.
- **Test Additions:**
- Extended ZNS importer tests with new scenarios for qualification parsing
stefan