feat(db): add SourceNode column to Notifications

src/ScadaLink.ConfigurationDatabase/Migrations/20260523201950_AddNotificationSourceNode.cs

@@ -0,0 +1,41 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ScadaLink.ConfigurationDatabase.Migrations
+{
+    /// <summary>
+    /// Adds the <c>SourceNode</c> column to the central <c>Notifications</c> table (#21,
+    /// SourceNode-stamping). <c>SourceNode</c> identifies the cluster node that produced the
+    /// notification (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.
+    ///
+    /// 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>. Unlike
+    /// <c>AuditLog</c>, the <c>Notifications</c> table is NOT partitioned, so a plain
+    /// <c>ADD</c> is fine. No index — Notification Outbox KPIs are per-site, not per-node,
+    /// on this table; <c>SourceNode</c> is only echoed onto <c>NotifyDeliver</c> audit rows
+    /// (#23) for cross-row correlation. Historical rows stay <c>NULL</c>.
+    /// </summary>
+    public partial class AddNotificationSourceNode : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "SourceNode",
+                table: "Notifications",
+                type: "varchar(64)",
+                maxLength: 64,
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "SourceNode",
+                table: "Notifications");
+        }
+    }
+}

src/ScadaLink.ConfigurationDatabase/Migrations/ScadaLinkDbContextModelSnapshot.cs

@@ -820,6 +820,10 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                         .HasMaxLength(200)
                         .HasColumnType("nvarchar(200)");
 
+                    b.Property<string>("SourceNode")
+                        .HasMaxLength(64)
+                        .HasColumnType("varchar(64)");
+
                     b.Property<string>("SourceScript")
                         .HasMaxLength(200)
                         .HasColumnType("nvarchar(200)");