Migrate to MyBatis with proper Controller→Service→Mapper layering

- Add MyBatis Spring Boot Starter with XML mappers and domain classes
- Create 9 mapper interfaces + XML: Restaurant, Video, Channel, Review,
  User, Stats, DaemonConfig, Search, Vector
- Create 10 domain classes with Lombok: Restaurant, VideoSummary,
  VideoDetail, VideoRestaurantLink, Channel, Review, UserInfo,
  DaemonConfig, SiteVisitStats, VectorSearchResult
- Create 7 new service classes: RestaurantService, VideoService,
  ChannelService, ReviewService, UserService, StatsService,
  DaemonConfigService
- Refactor all controllers to be thin (HTTP + auth only), delegating
  business logic to services
- Refactor SearchService, PipelineService, DaemonScheduler, AuthService,
  YouTubeService to use mappers/services instead of JDBC/repositories
- Add Jackson SNAKE_CASE property naming for consistent API responses
- Add ClobTypeHandler for Oracle CLOB→String in MyBatis
- Add IdGenerator utility for centralized UUID generation
- Delete old repository/ package (6 files), JdbcConfig, LowerCaseKeyAdvice
- VectorService retains JDBC for Oracle VECTOR type support

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

backend-java/build.gradle

@@ -22,6 +22,9 @@ dependencies {
     implementation 'org.springframework.boot:spring-boot-starter-web'
     implementation 'org.springframework.boot:spring-boot-starter-jdbc'
     implementation 'org.springframework.boot:spring-boot-starter-security'
+
+    // MyBatis
+    implementation 'org.mybatis.spring.boot:mybatis-spring-boot-starter:3.0.4'
     implementation 'org.springframework.boot:spring-boot-starter-data-redis'
     implementation 'org.springframework.boot:spring-boot-starter-validation'