fix: DB_PATH 从 backend-python/ 改为 backend/sqlite3.db

- .env: DB_PATH=../backend/sqlite3.db
- config.go default: ../backend/sqlite3.db
- 删除根目录和 backend/ 下 0 字节残留空 db 文件
- 全新初始化验证: 4用户 + 1审批流程 + 4登录均通过

backend/config/config.go

@@ -23,7 +23,7 @@ func Init() {
 
 	C = Config{
 		Port:      getEnv("PORT", "9000"),
-		DBPath:    getEnv("DB_PATH", "../backend-python/sqlite3.db"),
+		DBPath:    getEnv("DB_PATH", "../backend/sqlite3.db"),
 		JWTSecret: getEnv("JWT_SECRET", "sql-platform-secret-key-change-in-production"),
 		JWTExpire: getEnvInt("JWT_EXPIRE_HOURS", 24),
 		GhOstPath: getEnv("GH_OST_PATH", "gh-ost"),