feat(horses-service): remove integration tests and obsolete configurations

- Deleted outdated integration test classes (`HorseServiceIntegrationTest`, `TransactionalContextTest`, and others) and test resources (`logback-test.xml`).
- Removed obsolete Gradle dependencies related to these tests and revised project module references.
- Simplified `DomPferd` domain model with minor refactorings for serialization and validation.

Signed-off-by: Stefan Mogeritsch <stefan.mo.co@gmail.com>

backend/services/officials/officials-service/build.gradle.kts

@@ -13,8 +13,8 @@ dependencies {
   implementation(projects.platform.platformDependencies)
   implementation(projects.core.coreDomain)
   implementation(projects.core.coreUtils)
-  implementation(projects.officials.officialsDomain)
-  implementation(projects.officials.officialsInfrastructure)
+  implementation(projects.backend.services.officials.officialsDomain)
+  implementation(projects.backend.services.officials.officialsInfrastructure)
 
   implementation(libs.spring.boot.starter.web)
   implementation(libs.spring.boot.starter.validation)
@@ -23,6 +23,7 @@ dependencies {
   implementation(libs.exposed.core)
   implementation(libs.exposed.dao)
   implementation(libs.exposed.jdbc)
+  implementation(libs.exposed.migration.jdbc)
   implementation(libs.exposed.kotlin.datetime)
   implementation(libs.hikari.cp)
   runtimeOnly(libs.postgresql.driver)