feat: 添加会议转录文件服务

- 新增 `MeetingTranscriptFileServiceImpl` 实现会议转录文件的初始化和导出功能
- 定义 `MeetingTranscriptExportResult` 数据传输对象，用于封装导出结果
- 定义 `MeetingTranscriptFileService` 接口，提供初始化和导出会议转录文件的方法

backend/src/main/java/com/imeeting/dto/biz/MeetingTranscriptExportResult.java

@@ -0,0 +1,12 @@
+package com.imeeting.dto.biz;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+@Data
+@AllArgsConstructor
+public class MeetingTranscriptExportResult {
+    private byte[] content;
+    private String contentType;
+    private String fileName;
+}

backend/src/main/java/com/imeeting/service/biz/MeetingTranscriptFileService.java

@@ -0,0 +1,11 @@
+package com.imeeting.service.biz;
+
+import com.imeeting.dto.biz.MeetingTranscriptExportResult;
+import com.imeeting.dto.biz.MeetingVO;
+import com.imeeting.entity.biz.Meeting;
+
+public interface MeetingTranscriptFileService {
+    void initializeTranscriptFileIfAbsent(Long meetingId);
+
+    MeetingTranscriptExportResult exportTranscript(Meeting meeting, MeetingVO meetingDetail);
+}

backend/src/main/java/com/imeeting/service/biz/impl/MeetingTranscriptFileServiceImpl.java

@@ -0,0 +1,180 @@
+package com.imeeting.service.biz.impl;
+
+import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
+import com.imeeting.dto.biz.MeetingTranscriptExportResult;
+import com.imeeting.dto.biz.MeetingVO;
+import com.imeeting.entity.biz.Meeting;
+import com.imeeting.entity.biz.MeetingTranscript;
+import com.imeeting.mapper.biz.MeetingMapper;
+import com.imeeting.mapper.biz.MeetingTranscriptMapper;
+import com.imeeting.service.biz.MeetingTranscriptFileService;
+import lombok.RequiredArgsConstructor;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.stereotype.Service;
+
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.time.LocalDateTime;
+import java.time.format.DateTimeFormatter;
+import java.util.List;
+
+@Service
+@RequiredArgsConstructor
+public class MeetingTranscriptFileServiceImpl implements MeetingTranscriptFileService {
+
+    private static final String TRANSCRIPT_RELATIVE_PATH_TEMPLATE = "meetings/%s/transcripts/current.md";
+    private static final String CONTENT_TYPE = "text/markdown; charset=UTF-8";
+    private static final DateTimeFormatter DATE_TIME_FORMATTER = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
+
+    private final MeetingMapper meetingMapper;
+    private final MeetingTranscriptMapper meetingTranscriptMapper;
+
+    @Value("${unisbase.app.upload-path}")
+    private String uploadPath;
+
+    @Override
+    public void initializeTranscriptFileIfAbsent(Long meetingId) {
+        if (meetingId == null) {
+            return;
+        }
+        Path transcriptPath = buildTranscriptPath(meetingId);
+        if (Files.exists(transcriptPath)) {
+            return;
+        }
+
+        Meeting meeting = meetingMapper.selectById(meetingId);
+        if (meeting == null) {
+            return;
+        }
+        writeTranscriptFile(meeting, null);
+    }
+
+    @Override
+    public MeetingTranscriptExportResult exportTranscript(Meeting meeting, MeetingVO meetingDetail) {
+        if (meeting == null || meeting.getId() == null) {
+            throw new RuntimeException("会议不存在");
+        }
+        byte[] content = writeTranscriptFile(meeting, meetingDetail);
+        String safeTitle = sanitizeFileName(
+                meetingDetail != null ? meetingDetail.getTitle() : meeting.getTitle(),
+                "meeting-transcript-" + meeting.getId()
+        );
+        return new MeetingTranscriptExportResult(content, CONTENT_TYPE, safeTitle + "-Transcript.md");
+    }
+
+    private byte[] writeTranscriptFile(Meeting meeting, MeetingVO meetingDetail) {
+        try {
+            Path transcriptPath = buildTranscriptPath(meeting.getId());
+            Path parent = transcriptPath.getParent();
+            if (parent != null) {
+                Files.createDirectories(parent);
+            }
+            String markdown = buildTranscriptMarkdown(meeting, meetingDetail);
+            Files.writeString(transcriptPath, markdown, StandardCharsets.UTF_8);
+            return markdown.getBytes(StandardCharsets.UTF_8);
+        } catch (IOException ex) {
+            throw new RuntimeException("写入会议转录文件失败", ex);
+        }
+    }
+
+    private String buildTranscriptMarkdown(Meeting meeting, MeetingVO meetingDetail) {
+        List<MeetingTranscript> transcripts = meetingTranscriptMapper.selectList(new LambdaQueryWrapper<MeetingTranscript>()
+                .eq(MeetingTranscript::getMeetingId, meeting.getId())
+                .orderByAsc(MeetingTranscript::getSortOrder)
+                .orderByAsc(MeetingTranscript::getStartTime)
+                .orderByAsc(MeetingTranscript::getId));
+
+        String title = firstNonBlank(meetingDetail != null ? meetingDetail.getTitle() : null, meeting.getTitle(), "未命名会议");
+        String hostName = firstNonBlank(meetingDetail != null ? meetingDetail.getHostName() : null, meeting.getHostName(), "未知");
+        String meetingTime = formatDateTime(meetingDetail != null ? meetingDetail.getMeetingTime() : meeting.getMeetingTime());
+
+        StringBuilder builder = new StringBuilder();
+        builder.append("# ").append(title).append(" 会议转录\n\n");
+        builder.append("- 会议时间：").append(meetingTime).append("\n");
+        builder.append("- 主持人：").append(hostName).append("\n");
+        builder.append("- 导出时间：").append(formatDateTime(LocalDateTime.now())).append("\n\n");
+        builder.append("## 转录正文\n\n");
+
+        if (transcripts.isEmpty()) {
+            builder.append("_当前暂无转录内容_\n");
+            return builder.toString();
+        }
+
+        for (MeetingTranscript transcript : transcripts) {
+            String speaker = firstNonBlank(transcript.getSpeakerName(), transcript.getSpeakerId(), "未知发言人");
+            builder.append("- ");
+            String timeRange = buildTimeRange(transcript.getStartTime(), transcript.getEndTime());
+            if (!timeRange.isBlank()) {
+                builder.append(timeRange).append(' ');
+            }
+            builder.append(speaker).append("：").append(normalizeTranscriptContent(transcript.getContent())).append("\n");
+        }
+        return builder.toString();
+    }
+
+    private Path buildTranscriptPath(Long meetingId) {
+        String basePath = uploadPath.endsWith("/") || uploadPath.endsWith("\\") ? uploadPath : uploadPath + "/";
+        String relativePath = TRANSCRIPT_RELATIVE_PATH_TEMPLATE.formatted(meetingId);
+        return Paths.get(basePath, relativePath.replace("\\", "/"));
+    }
+
+    private String buildTimeRange(Integer startTime, Integer endTime) {
+        if (startTime == null && endTime == null) {
+            return "";
+        }
+        String start = formatMillis(startTime);
+        String end = formatMillis(endTime);
+        if (start.isBlank()) {
+            return "[" + end + "]";
+        }
+        if (end.isBlank()) {
+            return "[" + start + "]";
+        }
+        return "[" + start + " - " + end + "]";
+    }
+
+    private String formatMillis(Integer millis) {
+        if (millis == null || millis < 0) {
+            return "";
+        }
+        int totalSeconds = millis / 1000;
+        int hours = totalSeconds / 3600;
+        int minutes = (totalSeconds % 3600) / 60;
+        int seconds = totalSeconds % 60;
+        return String.format("%02d:%02d:%02d", hours, minutes, seconds);
+    }
+
+    private String normalizeTranscriptContent(String content) {
+        if (content == null || content.isBlank()) {
+            return "";
+        }
+        return content.replace("\r\n", " ").replace('\n', ' ').trim();
+    }
+
+    private String sanitizeFileName(String value, String fallback) {
+        String normalized = value == null ? "" : value.replaceAll("[\\\\/:*?\"<>|\\r\\n]", "_").trim();
+        return normalized.isEmpty() ? fallback : normalized;
+    }
+
+    private String formatDateTime(LocalDateTime value) {
+        if (value == null) {
+            return "未知";
+        }
+        return DATE_TIME_FORMATTER.format(value);
+    }
+
+    private String firstNonBlank(String... values) {
+        if (values == null) {
+            return "";
+        }
+        for (String value : values) {
+            if (value != null && !value.isBlank()) {
+                return value.trim();
+            }
+        }
+        return "";
+    }
+}