feat: 文传易匿名临时文件传输站初始版本

- 后端 .NET 8 + FreeSql + 阿里云 OSS，支持共享/私密/标签三种文件模式
- 前端 Vue3 + Vite + TDesign，上传/取件/管理三页
- 取件码/二维码/海报合成（含保存二维码为图片）
- deploy/ 部署文档与一键 FTP 发布脚本
- appsettings.json 含真实凭据，已 gitignore 不入库

backend/WenChuanyi.Api/Services/CodeGeneratorService.cs

@@ -0,0 +1,57 @@
+using WenChuanyi.Api.Models;
+
+namespace WenChuanyi.Api.Services;
+
+/// <summary>取件码/管理码生成与查重（冲突自动重生成）</summary>
+public class CodeGeneratorService
+{
+    // 排除易混淆 0/O/1/I/l
+    private const string AdminChars = "ABCDEFGHJKMNPQRSTUVWXYZabcdefghjkmnpqrstuvwxyz23456789";
+
+    private readonly IFreeSql _fsql;
+
+    public CodeGeneratorService(IFreeSql fsql) => _fsql = fsql;
+
+    /// <summary>生成 length 位纯数字取件码（全局唯一）</summary>
+    public async Task<string> GeneratePickCodeAsync(int length)
+    {
+        for (var i = 0; i < 30; i++)
+        {
+            var code = GenerateDigits(length);
+            var exists = await _fsql.Select<FileItem>().Where(f => f.PickCode == code).AnyAsync();
+            if (!exists)
+            {
+                return code;
+            }
+        }
+        throw new InvalidOperationException("取件码生成冲突次数过多，请重试");
+    }
+
+    /// <summary>生成 8 位字母数字管理码（全局唯一）</summary>
+    public async Task<string> GenerateAdminCodeAsync()
+    {
+        for (var i = 0; i < 30; i++)
+        {
+            var code = new string(Enumerable.Range(0, 8)
+                .Select(_ => AdminChars[Random.Shared.Next(AdminChars.Length)]).ToArray());
+            var exists = await _fsql.Select<FileItem>().Where(f => f.AdminCode == code).AnyAsync();
+            if (!exists)
+            {
+                return code;
+            }
+        }
+        throw new InvalidOperationException("管理码生成冲突次数过多，请重试");
+    }
+
+    private static string GenerateDigits(int length)
+    {
+        var first = Random.Shared.Next(1, 10); // 首位非 0
+        var sb = new System.Text.StringBuilder();
+        sb.Append(first);
+        for (var i = 1; i < length; i++)
+        {
+            sb.Append(Random.Shared.Next(0, 10));
+        }
+        return sb.ToString();
+    }
+}