refactor(core): Stabilize and Refactor Shared Kernel

This commit introduces a comprehensive refactoring and stabilization of the core module, establishing a robust and well-tested foundation (Shared Kernel) for all other services.

The module has been thoroughly analyzed, cleaned up, and equipped with a professional-grade test suite.

Architectural Refinements:
- Slimmed down `core-domain` to be a true, minimal Shared Kernel by removing all domain-specific enums (`PferdeGeschlechtE`, `SparteE`, etc.). This enforces loose coupling between feature modules.
- The only remaining enum is `DatenQuelleE`, which is a cross-cutting concern.

Code Refactoring & Improvements:
- Refactored the configuration loading by introducing a `ConfigLoader` class. This decouples the `AppConfig` data classes from the loading mechanism, significantly improving the testability of components that rely on configuration.
- Unified the previously duplicated `ValidationResult` and `ValidationError` classes into a single, serializable source of truth, ensuring consistent error reporting across all APIs.

Testing Enhancements:
- Introduced a comprehensive test suite for the core module, bringing it to a production-ready quality standard.
- Implemented the "gold standard" for database testing by replacing the previous H2 approach with **Testcontainers**. The `DatabaseFactory` is now tested against a real, ephemeral PostgreSQL container, guaranteeing 100% production parity.
- Added robust unit and integration tests for critical components, including the new `ConfigLoader`, all custom `Serializers`, and the `ApiResponse` logic.
- Fixed all compilation and runtime errors in the test suite, resulting in a successful `./gradlew clean build`.

gradle/libs.versions.toml

@@ -55,6 +55,7 @@ caffeine = "3.1.8"
 reactorKafka = "1.3.22"
 jackson = "2.17.0"
 jakartaAnnotation = "2.1.1"
+roomCommonJvm = "2.7.2"
 
 [libraries]
 # --- Platform BOMs (Bill of Materials) ---
@@ -174,6 +175,7 @@ assertj-core = { module = "org.assertj:assertj-core", version.ref = "assertj" }
 testcontainers-core = { module = "org.testcontainers:testcontainers", version.ref = "testcontainers" }
 testcontainers-junit-jupiter = { module = "org.testcontainers:junit-jupiter", version.ref = "testcontainers" }
 testcontainers-postgresql = { module = "org.testcontainers:postgresql", version.ref = "testcontainers" }
+room-common-jvm = { group = "androidx.room", name = "room-common-jvm", version.ref = "roomCommonJvm" }
 
 [bundles]
 # OPTIMIERUNG: Bündelt gängige Abhängigkeitsgruppen.