fix(delegated): tenant-template membership write failed (Edm.DateTime out of range)

AddTenantToTemplate returned 500 on the first add: the membership-row write
threw because TenantTemplateEntity.CreatedDate was left at default(DateTime)
(0001-01-01), which is below Azure Table Storage's Edm.DateTime minimum (1601).
The meta row wrote fine only because Create stamps CreatedDate=UtcNow.

Make CreatedDate nullable so membership rows omit it (null is valid); meta rows
still carry it. Read-back maps null -> default. Confirmed via storage: the
affected partition had only the meta row, no membership row.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/Backend/AutopilotMonitor.Functions/DataAccess/TableStorage/TableAdminRepository.cs

@@ -828,7 +828,7 @@ namespace AutopilotMonitor.Functions.DataAccess.TableStorage
             {
                 template.Name = entity.Name;
                 template.CreatedBy = entity.CreatedBy;
-                template.CreatedAt = entity.CreatedDate;
+                template.CreatedAt = entity.CreatedDate ?? default;
                 return true;
             }
 

src/Backend/AutopilotMonitor.Functions/Services/DelegatedAdminService.cs

@@ -414,8 +414,12 @@ public class TenantTemplateEntity : ITableEntity
     /// <summary>UPN of the GlobalAdmin who created the template (meta row only).</summary>
     public string CreatedBy { get; set; } = string.Empty;
 
-    /// <summary>When the template was created (meta row only).</summary>
-    public DateTime CreatedDate { get; set; }
+    /// <summary>
+    /// When the template was created (meta row only). NULLABLE on purpose: membership rows leave it unset, and
+    /// Azure Table Storage rejects a default(DateTime) (0001-01-01) — below the Edm.DateTime minimum of 1601 —
+    /// which would make every membership-row write throw. A null DateTime? is simply omitted (valid).
+    /// </summary>
+    public DateTime? CreatedDate { get; set; }
 
     /// <summary>The managed tenant ID (lowercase) — denormalized copy of RowKey on membership rows; empty on meta.</summary>
     public string TenantId { get; set; } = string.Empty;