feat:【站内信】

backend/src/main/java/com/openisle/repository/MessageConversationRepository.java

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+package com.openisle.repository;
+
+import com.openisle.model.MessageConversation;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+import com.openisle.model.User;
+import java.util.List;
+
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
+
+import java.util.Optional;
+import com.openisle.model.User;
+import java.util.List;
+
+@Repository
+public interface MessageConversationRepository extends JpaRepository<MessageConversation, Long> {
+    @Query("SELECT c FROM MessageConversation c JOIN c.participants p1 JOIN c.participants p2 WHERE p1.user = :user1 AND p2.user = :user2")
+    Optional<MessageConversation> findConversationByUsers(@Param("user1") User user1, @Param("user2") User user2);
+    
+    @Query("SELECT DISTINCT c FROM MessageConversation c " +
+           "JOIN c.participants p " +
+           "LEFT JOIN FETCH c.lastMessage lm " +
+           "LEFT JOIN FETCH lm.sender " +
+           "LEFT JOIN FETCH c.participants cp " +
+           "LEFT JOIN FETCH cp.user " +
+           "WHERE p.user.id = :userId " +
+           "ORDER BY COALESCE(lm.createdAt, c.createdAt) DESC")
+    List<MessageConversation> findConversationsByUserIdOrderByLastMessageDesc(@Param("userId") Long userId);
+}

backend/src/main/java/com/openisle/repository/MessageParticipantRepository.java

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+package com.openisle.repository;
+
+import com.openisle.model.MessageParticipant;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+import java.util.List;
+import java.util.Optional;
+
+@Repository
+public interface MessageParticipantRepository extends JpaRepository<MessageParticipant, Long> {
+    Optional<MessageParticipant> findByConversationIdAndUserId(Long conversationId, Long userId);
+    List<MessageParticipant> findByUserId(Long userId);
+}

backend/src/main/java/com/openisle/repository/MessageRepository.java

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+package com.openisle.repository;
+
+import com.openisle.model.Message;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.domain.Page;
+import org.springframework.data.domain.Pageable;
+import org.springframework.stereotype.Repository;
+
+import java.util.List;
+
+@Repository
+public interface MessageRepository extends JpaRepository<Message, Long> {
+    List<Message> findByConversationIdOrderByCreatedAtAsc(Long conversationId);
+
+    Page<Message> findByConversationId(Long conversationId, Pageable pageable);
+
+    long countByConversationIdAndCreatedAtAfter(Long conversationId, java.time.LocalDateTime createdAt);
+    
+    // 只计算不是指定用户发送的消息（即别人发给当前用户的消息）
+    long countByConversationIdAndCreatedAtAfterAndSenderIdNot(Long conversationId, java.time.LocalDateTime createdAt, Long senderId);
+}