refactor: migrate to YAML config and add Maven dev/prod profiles

- Replace application.properties with application.yaml (base/prod config)
  and application-dev.yaml (dev overrides: show-sql=true)
- Add Maven 'dev' profile (activeByDefault) and 'prod' profile to pom.xml;
  spring-boot:run picks up the active Spring profile automatically
- Guard DataInitializer.initData with @Profile("dev") so test data is
  never seeded in production

Local dev: ./mvnw spring-boot:run (dev profile active by default)
Production: SPRING_PROFILES_ACTIVE env var controls the Spring profile;
            Maven profiles are irrelevant for the packaged JAR.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/config/DataInitializer.java

@@ -16,6 +16,7 @@ import org.springframework.beans.factory.annotation.Value;
 import org.springframework.boot.CommandLineRunner;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Profile;
 import org.springframework.security.crypto.password.PasswordEncoder;
 
 import java.time.LocalDate;
@@ -67,6 +68,7 @@ public class DataInitializer {
     }
 
     @Bean
+    @Profile("dev")
     public CommandLineRunner initData(PersonRepository personRepo,
             DocumentRepository docRepo,
             UserGroupRepository groupRepo) {