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/src/main/resources/application.yaml

@@ -1,6 +1,15 @@
 spring:
   application:
     name: mail-service
+  datasource:
+    url: jdbc:h2:mem:maildb;DB_CLOSE_DELAY=-1
+    driver-class-name: org.h2.Driver
+    username: sa
+    password: ""
+  h2:
+    console:
+      enabled: true
+      path: /h2-console
   mail:
     host: ${MAIL_HOST:imap.world4you.com}
     port: ${MAIL_PORT:993}