Add CustDocBase64 entity and integrate into DbContext

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.

src/core/DigitalData.TaskFlow.Domain/Entities/CustDocBase64.cs

@@ -0,0 +1,31 @@
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace DigitalData.TaskFlow.Domain.Entities;
+
+[Table("TBCUST_DOC_BASE64", Schema = "dbo")]
+public sealed class CustDocBase64
+{
+    [Column("ProcessID", TypeName = "bigint")]
+    public long? ProcessId { get; set; }
+
+    [Column("DocID", TypeName = "bigint")]
+    public long? DocId { get; set; }
+
+    [Column("Doctype", TypeName = "nvarchar(200)")]
+    public string? DocType { get; set; }
+
+    [Column("GroupType", TypeName = "nvarchar(200)")]
+    public string? GroupType { get; set; }
+
+    [Column("filename", TypeName = "nvarchar(500)")]
+    public string? FileName { get; set; }
+
+    [Column("filebase64", TypeName = "varchar(max)")]
+    public string? FileBase64 { get; set; }
+
+    [Column("ADDED_WHEN", TypeName = "datetime")]
+    public DateTime AddedWhen { get; set; }
+
+    [Column("thumbnailbase64", TypeName = "varchar(max)")]
+    public string? ThumbnailBase64 { get; set; }
+}

src/infrastructure/DigitalData.TaskFlow.Infrastructure/Persistence/TaskFlowDbContext.cs

@@ -1,3 +1,4 @@
+using DigitalData.TaskFlow.Domain.Entities;
 using DigitalData.TaskFlow.Infrastructure.Persistence.Models;
 using Microsoft.EntityFrameworkCore;
 
@@ -13,6 +14,11 @@ public sealed class TaskFlowDbContext(DbContextOptions<TaskFlowDbContext> option
     /// </summary>
     public DbSet<WorkflowListingReadModel> WorkflowListingReadModels => Set<WorkflowListingReadModel>();
 
+    /// <summary>
+    /// Gets the keyless entity mapped to TBCUST_DOC_BASE64.
+    /// </summary>
+    public DbSet<CustDocBase64> CustDocBase64 => Set<CustDocBase64>();
+
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
         base.OnModelCreating(modelBuilder);
@@ -22,5 +28,11 @@ public sealed class TaskFlowDbContext(DbContextOptions<TaskFlowDbContext> option
             entity.HasNoKey();
             entity.ToView(null);
         });
+
+        modelBuilder.Entity<CustDocBase64>(entity =>
+        {
+            entity.HasNoKey();
+            entity.ToTable("TBCUST_DOC_BASE64", "dbo");
+        });
     }
 }