feat(mail-service): introduce persistence and REST support for Nennungen

- Added `NennungRepository` with methods for saving, updating status, and retrieving entries.
- Created `NennungController` to expose REST endpoints for Nennungen.
- Defined `NennungTable` schema with relevant fields and indices.
- Extended `MailPollingService` to parse incoming emails into `NennungEntity` and persist them.
- Updated `build.gradle.kts` with database dependencies and H2 configuration for local dev.
- Refined frontend layout in `OnlineNennungFormular` for improved usability and responsiveness.

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

@@ -11,6 +11,7 @@ springBoot {
 
 dependencies {
   // Interne Module
+  implementation(platform(projects.platform.platformBom))
   implementation(projects.platform.platformDependencies)
   implementation(projects.core.coreUtils)
   implementation(projects.core.coreDomain)
@@ -20,7 +21,20 @@ dependencies {
   implementation(libs.spring.boot.starter.validation)
   implementation(libs.spring.boot.starter.actuator)
   implementation(libs.spring.boot.starter.mail)
+  implementation(libs.spring.boot.starter.jdbc)
   implementation(libs.jackson.module.kotlin)
+  implementation(libs.jackson.datatype.jsr310)
+
+  // Database & Exposed
+  implementation(libs.exposed.core)
+  implementation(libs.exposed.dao)
+  implementation(libs.exposed.jdbc)
+  implementation(libs.exposed.java.time)
+  implementation(libs.exposed.json)
+  implementation(libs.exposed.kotlin.datetime)
+  implementation(libs.h2.driver)
+  implementation(libs.postgresql.driver)
+  implementation(libs.hikari.cp)
   implementation(libs.spring.cloud.starter.consul.discovery)
   implementation(libs.micrometer.tracing.bridge.brave)
   implementation(libs.zipkin.reporter.brave)