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.Commons/Entities/Templates/Template.cs

@@ -12,6 +12,21 @@ public class Template
     public ICollection<TemplateScript> Scripts { get; set; } = new List<TemplateScript>();
     public ICollection<TemplateComposition> Compositions { get; set; } = new List<TemplateComposition>();
 
+    /// <summary>
+    /// True when this template was auto-derived to back a TemplateComposition
+    /// slot. Derived templates inherit from a base (see <see cref="ParentTemplateId"/>),
+    /// are owned by their composition row (see <see cref="OwnerCompositionId"/>),
+    /// and are hidden from the main template tree by default.
+    /// </summary>
+    public bool IsDerived { get; set; }
+
+    /// <summary>
+    /// Back-reference to the <see cref="TemplateComposition"/> that owns this
+    /// derived template. Non-null only when <see cref="IsDerived"/>; cascade-
+    /// delete when the composition is removed. Always null on base templates.
+    /// </summary>
+    public int? OwnerCompositionId { get; set; }
+
     public Template(string name)
     {
         Name = name ?? throw new ArgumentNullException(nameof(name));

src/ScadaLink.Commons/Entities/Templates/TemplateAttribute.cs

@@ -13,6 +13,21 @@ public class TemplateAttribute
     public string? Description { get; set; }
     public string? DataSourceReference { get; set; }
 
+    /// <summary>
+    /// True when this row was copied from the base template and has not been
+    /// overridden on the derived template. Changes to the base flow downward
+    /// for inherited rows; an explicit override flips this to false.
+    /// Always false on base (non-derived) templates.
+    /// </summary>
+    public bool IsInherited { get; set; }
+
+    /// <summary>
+    /// Set on a base attribute. When true, derived templates may not override
+    /// the value — the row is rendered readonly with a 🔒 in the derived UI,
+    /// and any attempt to update it through the API is rejected.
+    /// </summary>
+    public bool LockedInDerived { get; set; }
+
     public TemplateAttribute(string name)
     {
         Name = name ?? throw new ArgumentNullException(nameof(name));

src/ScadaLink.Commons/Entities/Templates/TemplateScript.cs

@@ -13,6 +13,21 @@ public class TemplateScript
     public string? ReturnDefinition { get; set; }
     public TimeSpan? MinTimeBetweenRuns { get; set; }
 
+    /// <summary>
+    /// True when this row was copied from the base template and has not been
+    /// overridden on the derived template. Changes to the base flow downward
+    /// for inherited rows; an explicit override flips this to false.
+    /// Always false on base (non-derived) templates.
+    /// </summary>
+    public bool IsInherited { get; set; }
+
+    /// <summary>
+    /// Set on a base script. When true, derived templates may not override
+    /// the script body — the row is rendered readonly with a 🔒 in the derived
+    /// UI, and any attempt to update it through the API is rejected.
+    /// </summary>
+    public bool LockedInDerived { get; set; }
+
     public TemplateScript(string name, string code)
     {
         Name = name ?? throw new ArgumentNullException(nameof(name));