IA-1008 : refactor: migrate userId to String and enrich assistant model with subject resolution

Summary

This MR migrates userId from Long to String across all layers of the assistant-japprends microservice, enriches the ChildExerciseAssistant model with subject/chapter/exercise metadata resolved at mapping time, and adds minor infrastructure improvements.

Changes

Dependency

• Bump brainboost-common 0.0.103-RELEASE → 0.0.104-SNAPSHOT

Infrastructure

• Add @EnableJpaAuditing to Application to enable JPA audit timestamps
• Expose RestTemplate as a Spring bean (required by the new mapper)
• Add subject-management.url property to application.yml

Breaking refactor — userId: Long → String

All layers updated consistently:

• DTOs: ChildExerciseAssistantDTO, InteractionExerciceDTO, ResponseHistoryDTO, ProcessingTaskDTO, RagQueryRequest
• Service interfaces & impls: RAGService, OCRService, SpeechService, ChildExerciseAssistantService, InteractionExerciceService, MultimediaProcessingService and all their implementations
• Controllers: ChildExerciseAssistantController, InteractionExerciceController

Assistant model enrichment

• ChildExerciseAssistantDTO gains: uuid, name, description, subjectId, chapterId, exerciseId, subjectName, chapterName, exerciseName
• ChildExerciseAssistantMapper rewritten to resolve human-readable names from subject-management service via RestTemplate at mapping time
• Added SubjectServiceClient interface + SubjectNameDTO, ChapterNameDTO, ExerciseNameDTO client DTOs

Repository

• ChildExerciseAssistantRepository: findByUserId migrated to String, in-memory search replaced with a proper JPQL @Query on userId / educationLevel / status
• ResponseHistoryRepository: simplified to a single findByUserIdOrderByDateResponseDesc(String) method

Interaction service

• save() and evaluateWithMedia() annotated with @Transactional
• evaluate() simplified (status management only, no direct RAG call)