feat(templates): phase 1 — derived-template schema (additive)

Phase 1 of the design at
docs/plans/2026-05-12-derive-on-compose-design.md.

Additive schema only — no behavior changes. Existing data and code
paths continue to work; subsequent phases will start writing the
new fields.

Template gains:
  IsDerived            true when this row was auto-created to back
                       a composition slot
  OwnerCompositionId   back-ref to the owning TemplateComposition
                       (plain int, not an EF nav property — managed
                       by TemplateService for cascade-delete)

TemplateAttribute / TemplateScript each gain:
  IsInherited          row copied from base and not yet overridden;
                       changes to the base flow downward
  LockedInDerived      on a base, blocks derived from overriding;
                       enforced at the service layer in later phases

EF Core migration AddDerivedTemplateFields adds four columns:
  Templates.IsDerived              bit NOT NULL DEFAULT 0
  Templates.OwnerCompositionId     int NULL
  TemplateAttributes.IsInherited   bit NOT NULL DEFAULT 0
  TemplateAttributes.LockedInDerived bit NOT NULL DEFAULT 0
  TemplateScripts.IsInherited      bit NOT NULL DEFAULT 0
  TemplateScripts.LockedInDerived  bit NOT NULL DEFAULT 0

Existing rows get the defaults. Tests across SiteRuntime / TemplateEngine
/ CentralUI suites stay green (129 / 199 / 159).

Next: phase 2 — wire AddCompositionAsync to derive on compose for
new compositions. Old data still flows the direct-reference path
until phase 3's migration script.

src/ScadaLink.ConfigurationDatabase/Migrations/20260512121446_AddDerivedTemplateFields.cs

@@ -0,0 +1,83 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ScadaLink.ConfigurationDatabase.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddDerivedTemplateFields : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<bool>(
+                name: "IsInherited",
+                table: "TemplateScripts",
+                type: "bit",
+                nullable: false,
+                defaultValue: false);
+
+            migrationBuilder.AddColumn<bool>(
+                name: "LockedInDerived",
+                table: "TemplateScripts",
+                type: "bit",
+                nullable: false,
+                defaultValue: false);
+
+            migrationBuilder.AddColumn<bool>(
+                name: "IsDerived",
+                table: "Templates",
+                type: "bit",
+                nullable: false,
+                defaultValue: false);
+
+            migrationBuilder.AddColumn<int>(
+                name: "OwnerCompositionId",
+                table: "Templates",
+                type: "int",
+                nullable: true);
+
+            migrationBuilder.AddColumn<bool>(
+                name: "IsInherited",
+                table: "TemplateAttributes",
+                type: "bit",
+                nullable: false,
+                defaultValue: false);
+
+            migrationBuilder.AddColumn<bool>(
+                name: "LockedInDerived",
+                table: "TemplateAttributes",
+                type: "bit",
+                nullable: false,
+                defaultValue: false);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "IsInherited",
+                table: "TemplateScripts");
+
+            migrationBuilder.DropColumn(
+                name: "LockedInDerived",
+                table: "TemplateScripts");
+
+            migrationBuilder.DropColumn(
+                name: "IsDerived",
+                table: "Templates");
+
+            migrationBuilder.DropColumn(
+                name: "OwnerCompositionId",
+                table: "Templates");
+
+            migrationBuilder.DropColumn(
+                name: "IsInherited",
+                table: "TemplateAttributes");
+
+            migrationBuilder.DropColumn(
+                name: "LockedInDerived",
+                table: "TemplateAttributes");
+        }
+    }
+}

src/ScadaLink.ConfigurationDatabase/Migrations/ScadaLinkDbContextModelSnapshot.cs

@@ -848,11 +848,17 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                     b.Property<int?>("FolderId")
                         .HasColumnType("int");
 
+                    b.Property<bool>("IsDerived")
+                        .HasColumnType("bit");
+
                     b.Property<string>("Name")
                         .IsRequired()
                         .HasMaxLength(200)
                         .HasColumnType("nvarchar(200)");
 
+                    b.Property<int?>("OwnerCompositionId")
+                        .HasColumnType("int");
+
                     b.Property<int?>("ParentTemplateId")
                         .HasColumnType("int");
 
@@ -935,9 +941,15 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                         .HasMaxLength(2000)
                         .HasColumnType("nvarchar(2000)");
 
+                    b.Property<bool>("IsInherited")
+                        .HasColumnType("bit");
+
                     b.Property<bool>("IsLocked")
                         .HasColumnType("bit");
 
+                    b.Property<bool>("LockedInDerived")
+                        .HasColumnType("bit");
+
                     b.Property<string>("Name")
                         .IsRequired()
                         .HasMaxLength(200)
@@ -1027,9 +1039,15 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                         .IsRequired()
                         .HasColumnType("nvarchar(max)");
 
+                    b.Property<bool>("IsInherited")
+                        .HasColumnType("bit");
+
                     b.Property<bool>("IsLocked")
                         .HasColumnType("bit");
 
+                    b.Property<bool>("LockedInDerived")
+                        .HasColumnType("bit");
+
                     b.Property<TimeSpan?>("MinTimeBetweenRuns")
                         .HasColumnType("time");