fix: clean up empty catch blocks in backend utilities

修复了后端工具文件中的 10 个空 catch 块：
- system-crypto.ts: 修复 1 个空 catch 块
- server-stats.ts: 修复 4 个空 catch 块
- auto-ssl-setup.ts: 修复 1 个空 catch 块
- ssh-key-utils.ts: 修复 4 个空 catch 块

所有空块都添加了描述性注释说明为何忽略错误。

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/backend/utils/auto-ssl-setup.ts

@@ -234,7 +234,9 @@ IP.3 = 0.0.0.0
     let envContent = "";
     try {
       envContent = await fs.readFile(this.ENV_FILE, "utf8");
-    } catch {}
+    } catch {
+      // File doesn't exist yet, will create with SSL config
+    }
 
     let updatedContent = envContent;
     let hasChanges = false;