feat(db): add SourceNode column to SiteCalls

src/ScadaLink.ConfigurationDatabase/Migrations/20260523202131_AddSiteCallSourceNode.cs

@@ -0,0 +1,42 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ScadaLink.ConfigurationDatabase.Migrations
+{
+    /// <summary>
+    /// Adds the <c>SourceNode</c> column to the central <c>SiteCalls</c> table (#22,
+    /// SourceNode-stamping). <c>SourceNode</c> identifies the cluster node that produced the
+    /// row (e.g. <c>node-a</c>, <c>central-a</c>) — ASCII-only, so <c>varchar(64)</c> not
+    /// <c>nvarchar</c>. <c>NULL</c> is valid for rows that pre-date this feature and for
+    /// reconciled rows from a retired node.
+    ///
+    /// The change is purely additive: <c>SourceNode varchar(64) NULL</c> is added with no
+    /// default, so the operation is a metadata-only <c>ALTER TABLE … ADD</c>. The
+    /// <c>SiteCalls</c> table is NOT partitioned (operational state, not audit), so a plain
+    /// <c>ADD</c> is fine. No index — Site Call Audit KPIs are per-site, not per-node, on
+    /// this table; <c>SourceNode</c> is operational metadata, never a query predicate here.
+    /// Historical rows stay <c>NULL</c>.
+    /// </summary>
+    public partial class AddSiteCallSourceNode : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "SourceNode",
+                table: "SiteCalls",
+                type: "varchar(64)",
+                maxLength: 64,
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "SourceNode",
+                table: "SiteCalls");
+        }
+    }
+}

src/ScadaLink.ConfigurationDatabase/Migrations/ScadaLinkDbContextModelSnapshot.cs

@@ -262,6 +262,10 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                     b.Property<int>("RetryCount")
                         .HasColumnType("int");
 
+                    b.Property<string>("SourceNode")
+                        .HasMaxLength(64)
+                        .HasColumnType("varchar(64)");
+
                     b.Property<string>("SourceSite")
                         .IsRequired()
                         .HasMaxLength(64)