docs: 生成项目文档并优化项目结构

- 生成完整的 README.md 项目文档
- 统一图片路径为相对引用
- 完善 .gitignore 配置，添加常见忽略项
- 清理未使用的临时文件和图片资源
- 删除废弃的脚本文件

backend/migrations/005_create_subcontractor_payment_infos.sql

@@ -1,14 +1,14 @@
 -- 创建分包商收款信息表
 CREATE TABLE IF NOT EXISTS subcontractor_payment_infos (
-  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  id SERIAL PRIMARY KEY,
   subcontractor_id INTEGER NOT NULL,
   account_name TEXT NOT NULL,
   bank_account TEXT NOT NULL,
   bank_name TEXT NOT NULL,
   qr_code TEXT,
   is_primary INTEGER DEFAULT 0,
-  created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
-  updated_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+  updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
   FOREIGN KEY (subcontractor_id) REFERENCES subcontractors(id) ON DELETE CASCADE
 );