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/resources/application.yaml

@@ -0,0 +1,42 @@
+spring:
+  application:
+    name: Familienarchiv
+
+  datasource:
+    url: ${SPRING_DATASOURCE_URL}
+    username: ${SPRING_DATASOURCE_USERNAME}
+    password: ${SPRING_DATASOURCE_PASSWORD}
+    driver-class-name: org.postgresql.Driver
+
+  jpa:
+    hibernate:
+      ddl-auto: none
+    properties:
+      hibernate:
+        dialect: org.hibernate.dialect.PostgreSQLDialect
+    show-sql: false
+
+  servlet:
+    multipart:
+      max-file-size: 50MB
+      max-request-size: 50MB
+
+app:
+  s3:
+    endpoint: ${S3_ENDPOINT}
+    access-key: ${S3_ACCESS_KEY}
+    secret-key: ${S3_SECRET_KEY}
+    bucket: ${S3_BUCKET_NAME}
+    region: ${S3_REGION}
+
+  admin:
+    username: ${APP_ADMIN_USERNAME:admin}
+    password: ${APP_ADMIN_PASSWORD:admin123}
+
+  import:
+    excel:
+      col:
+        filename: 0
+        date: 1
+        location: 2
+        transcription: 3