feat: replace site registration with database-driven site addressing

Central now resolves site Akka remoting addresses from the Sites DB table
(NodeAAddress/NodeBAddress) instead of relying on runtime RegisterSite
messages. Eliminates the race condition where sites starting before central
had their registration dead-lettered. Addresses are cached in
CentralCommunicationActor with 60s periodic refresh and on-demand refresh
when sites are added/edited/deleted via UI or CLI.

src/ScadaLink.ConfigurationDatabase/Migrations/20260318025613_AddSiteNodeAddresses.cs

@@ -0,0 +1,42 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+#pragma warning disable CA1814 // Prefer jagged arrays over multidimensional
+
+namespace ScadaLink.ConfigurationDatabase.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddSiteNodeAddresses : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "NodeAAddress",
+                table: "Sites",
+                type: "nvarchar(500)",
+                maxLength: 500,
+                nullable: true);
+
+            migrationBuilder.AddColumn<string>(
+                name: "NodeBAddress",
+                table: "Sites",
+                type: "nvarchar(500)",
+                maxLength: 500,
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "NodeAAddress",
+                table: "Sites");
+
+            migrationBuilder.DropColumn(
+                name: "NodeBAddress",
+                table: "Sites");
+        }
+    }
+}

src/ScadaLink.ConfigurationDatabase/Migrations/ScadaLinkDbContextModelSnapshot.cs

@@ -699,6 +699,24 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                             Id = 1,
                             LdapGroupName = "SCADA-Admins",
                             Role = "Admin"
+                        },
+                        new
+                        {
+                            Id = 2,
+                            LdapGroupName = "SCADA-Designers",
+                            Role = "Design"
+                        },
+                        new
+                        {
+                            Id = 3,
+                            LdapGroupName = "SCADA-Deploy-All",
+                            Role = "Deployment"
+                        },
+                        new
+                        {
+                            Id = 4,
+                            LdapGroupName = "SCADA-Deploy-SiteA",
+                            Role = "Deployment"
                         });
                 });
 
@@ -773,6 +791,14 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                         .HasMaxLength(200)
                         .HasColumnType("nvarchar(200)");
 
+                    b.Property<string>("NodeAAddress")
+                        .HasMaxLength(500)
+                        .HasColumnType("nvarchar(500)");
+
+                    b.Property<string>("NodeBAddress")
+                        .HasMaxLength(500)
+                        .HasColumnType("nvarchar(500)");
+
                     b.Property<string>("SiteIdentifier")
                         .IsRequired()
                         .HasMaxLength(100)