feat: cross-aspect project duplication + AEP convention/rule-engine spec

- content-svc: DuplicateProjectAsync clones full scene/element/colour graph
  (identical keys, new dimensions/aspect; AEP intentionally not copied;
  starts unpublished) + POST /v1/projects/{id}/duplicate.
- admin: «تکثیر» button + modal on each project row; aspects reduced to
  supported 16:9/1:1/9:16; free fps default 21 (clamped 1-60).
- docs/aep-template-convention.md: versioned (v1/v2) convention + rule-engine
  spec — modes, scene types, flatrender assembly, duration/fade model,
  fit-box, input types, expression-driven data flow, output spec.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/content/FlatRender.ContentSvc/Models/ProjectExtraModels.cs

@@ -0,0 +1,13 @@
+namespace FlatRender.ContentSvc.Models.Requests;
+
+/// <summary>Clone a project to a new aspect ratio. Only dimensions (+ optional name/container/
+/// resolution) change; all scene/element/colour keys are copied identically. Null fields inherit
+/// from the source project.</summary>
+public record DuplicateProjectRequest(
+    string? Aspect,
+    int? OriginalWidth,
+    int? OriginalHeight,
+    string? Resolution,
+    string? Name,
+    Guid? ContainerId
+);