refactor: 清理所有调试日志和死代码

- 🗑️ 删除未使用的依赖: net, dns 模块
- 🧹 将 console.log("[DEBUG]") 替换为 SystemLogDB.log()
- 🧹 清理分享验证相关的调试日志
- 🧹 清理数据库查询的调试日志
- 🧹 删除下载功能的调试日志
- ✅ 代码更专业，日志统一使用 SystemLogDB

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/database.js

@@ -441,14 +441,6 @@ const ShareDB = {
 
   // 根据分享码查找
   findByCode(shareCode) {
-    // 调试日志: findByCode 调用
-    const currentTime = db.prepare("SELECT datetime('now', 'localtime') as now").get();
-    console.log('[ShareDB.findByCode]', {
-      shareCode,
-      currentTime: currentTime.now,
-      timestamp: new Date().toISOString()
-    });
-
     const result = db.prepare(`
       SELECT s.*, u.username, u.oss_provider, u.oss_region, u.oss_access_key_id, u.oss_access_key_secret, u.oss_bucket, u.oss_endpoint, u.theme_preference
       FROM shares s
@@ -457,14 +449,6 @@ const ShareDB = {
         AND (s.expires_at IS NULL OR s.expires_at > datetime('now', 'localtime'))
     `).get(shareCode);
 
-    // 调试日志: SQL查询结果
-    console.log('[ShareDB.findByCode] SQL结果:', {
-      found: !!result,
-      shareCode: result?.share_code || null,
-      expires_at: result?.expires_at || null,
-      share_type: result?.share_type || null
-    });
-
     return result;
   },