fix(email): preserve draft send identity

Persist and export each draft's idempotency UUID so refresh and edits cannot create a fresh delivery identity. Add reversible provider-specific migration SQL.

JobTrackerApi/Migrations/20260810080858_AddEmailDraftClientRequestId.cs

@@ -0,0 +1,39 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace JobTrackerApi.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddEmailDraftClientRequestId : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            if (ActiveProvider.Contains("MySql", StringComparison.OrdinalIgnoreCase))
+            {
+                migrationBuilder.Sql("""
+                    ALTER TABLE `EmailDrafts`
+                    ADD `ClientRequestId` varchar(128) NOT NULL DEFAULT '';
+                    """);
+                return;
+            }
+
+            migrationBuilder.AddColumn<string>(
+                name: "ClientRequestId",
+                table: "EmailDrafts",
+                type: "TEXT",
+                maxLength: 128,
+                nullable: false,
+                defaultValue: "");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "ClientRequestId",
+                table: "EmailDrafts");
+        }
+    }
+}

JobTrackerApi/Migrations/JobTrackerContextModelSnapshot.cs

@@ -1108,6 +1108,11 @@ namespace JobTrackerApi.Migrations
                         .IsRequired()
                         .HasColumnType("TEXT");
 
+                    b.Property<string>("ClientRequestId")
+                        .IsRequired()
+                        .HasMaxLength(128)
+                        .HasColumnType("TEXT");
+
                     b.Property<DateTime>("CreatedAtUtc")
                         .HasColumnType("TEXT");