feat(templates): phase 3 — migrate existing compositions to derived

EF migration MigrateCompositionsToDerived. Aborts with a clear error if
any '<parent>.<slot>' derived name would collide with an existing
template. Otherwise it cursor-walks every TemplateComposition that still
points at a non-derived template:

  1. Insert a derived Template (name "<parent>.<slot>",
     ParentTemplateId=base, IsDerived=1, OwnerCompositionId=composition).
  2. Copy base attributes / scripts into the derived row with
     IsInherited=1, LockedInDerived=0.
  3. Repoint TemplateComposition.ComposedTemplateId at the new derived.

Idempotent: only touches compositions whose target is IsDerived=0, so
re-runs and freshly-created Phase 2 compositions are skipped.

Down() reverses by repointing compositions back to derived.ParentTemplateId
and dropping all derived templates (with cascade copy rows).

src/ScadaLink.ConfigurationDatabase/Migrations/20260512122746_MigrateCompositionsToDerived.cs

@@ -0,0 +1,117 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ScadaLink.ConfigurationDatabase.Migrations
+{
+    /// <inheritdoc />
+    public partial class MigrateCompositionsToDerived : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            // Re-shape every pre-Phase-2 TemplateComposition so it points at a
+            // newly created derived template ("<parent>.<slot>") that inherits
+            // from the original base. Attribute and script rows are copied with
+            // IsInherited=1; the composition's ComposedTemplateId is repointed.
+            //
+            // Idempotent: only rows whose target is still IsDerived=0 are touched.
+            // Aborts the migration if any derived name would collide with an
+            // existing template, so the operator can resolve manually.
+            migrationBuilder.Sql(@"
+SET NOCOUNT ON;
+
+DECLARE @collisions NVARCHAR(MAX) = (
+    SELECT STRING_AGG(owner.Name + N'.' + c.InstanceName, N', ')
+    FROM TemplateCompositions c
+    INNER JOIN Templates base_t ON base_t.Id = c.ComposedTemplateId
+    INNER JOIN Templates owner ON owner.Id = c.TemplateId
+    INNER JOIN Templates existing ON existing.Name = owner.Name + N'.' + c.InstanceName
+    WHERE base_t.IsDerived = 0
+);
+IF @collisions IS NOT NULL
+BEGIN
+    DECLARE @msg NVARCHAR(MAX) =
+        N'MigrateCompositionsToDerived: cannot create derived templates — these names already exist: '
+        + @collisions
+        + N'. Rename the conflicting templates and retry the migration.';
+    THROW 50000, @msg, 1;
+END
+
+DECLARE @CompId INT, @BaseId INT, @OwnerName NVARCHAR(200), @SlotName NVARCHAR(200);
+DECLARE @NewId INT, @NewName NVARCHAR(200);
+
+DECLARE map_cursor CURSOR FAST_FORWARD FOR
+    SELECT c.Id, c.ComposedTemplateId, owner.Name, c.InstanceName
+    FROM TemplateCompositions c
+    INNER JOIN Templates base_t ON base_t.Id = c.ComposedTemplateId
+    INNER JOIN Templates owner ON owner.Id = c.TemplateId
+    WHERE base_t.IsDerived = 0;
+
+OPEN map_cursor;
+FETCH NEXT FROM map_cursor INTO @CompId, @BaseId, @OwnerName, @SlotName;
+
+WHILE @@FETCH_STATUS = 0
+BEGIN
+    SET @NewName = @OwnerName + N'.' + @SlotName;
+
+    INSERT INTO Templates (Name, Description, ParentTemplateId, FolderId, IsDerived, OwnerCompositionId)
+    SELECT @NewName, b.Description, b.Id, NULL, 1, @CompId
+    FROM Templates b
+    WHERE b.Id = @BaseId;
+
+    SET @NewId = SCOPE_IDENTITY();
+
+    INSERT INTO TemplateAttributes
+        (TemplateId, Name, Value, DataType, IsLocked, Description, DataSourceReference, IsInherited, LockedInDerived)
+    SELECT @NewId, a.Name, a.Value, a.DataType, a.IsLocked, a.Description, a.DataSourceReference, 1, 0
+    FROM TemplateAttributes a
+    WHERE a.TemplateId = @BaseId;
+
+    INSERT INTO TemplateScripts
+        (TemplateId, Name, Code, IsLocked, TriggerType, TriggerConfiguration, ParameterDefinitions, ReturnDefinition, MinTimeBetweenRuns, IsInherited, LockedInDerived)
+    SELECT @NewId, s.Name, s.Code, s.IsLocked, s.TriggerType, s.TriggerConfiguration, s.ParameterDefinitions, s.ReturnDefinition, s.MinTimeBetweenRuns, 1, 0
+    FROM TemplateScripts s
+    WHERE s.TemplateId = @BaseId;
+
+    UPDATE TemplateCompositions
+    SET ComposedTemplateId = @NewId
+    WHERE Id = @CompId;
+
+    FETCH NEXT FROM map_cursor INTO @CompId, @BaseId, @OwnerName, @SlotName;
+END
+
+CLOSE map_cursor;
+DEALLOCATE map_cursor;
+");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            // Reverse: repoint each composition back to the derived template's
+            // base, then drop the derived templates (with their copied rows).
+            migrationBuilder.Sql(@"
+SET NOCOUNT ON;
+
+UPDATE c
+SET c.ComposedTemplateId = d.ParentTemplateId
+FROM TemplateCompositions c
+INNER JOIN Templates d ON d.Id = c.ComposedTemplateId
+WHERE d.IsDerived = 1
+  AND d.OwnerCompositionId = c.Id
+  AND d.ParentTemplateId IS NOT NULL;
+
+DELETE a FROM TemplateAttributes a
+INNER JOIN Templates t ON t.Id = a.TemplateId
+WHERE t.IsDerived = 1;
+
+DELETE s FROM TemplateScripts s
+INNER JOIN Templates t ON t.Id = s.TemplateId
+WHERE t.IsDerived = 1;
+
+DELETE FROM Templates WHERE IsDerived = 1;
+");
+        }
+    }
+}