Introduce a new entity class `CustDocBase64` mapped to the `TBCUST_DOC_BASE64` table in the `dbo` schema. This class includes properties for various document attributes. Update `TaskFlowDbContext` to include a `DbSet<CustDocBase64>` for querying and interacting with the table. Configure the entity in `OnModelCreating` to have no key and map it to the appropriate table and schema.