feat(presets): admin preset stories (premade example videos) end-to-end

Epic A — admins author premade example videos per template; users pick one
on the template detail page to start a pre-filled project.

Backend (content-svc):
- PresetStory DTOs + PresetStoryService (admin CRUD + public published-only
  filter via role check + soft-delete) + PresetStoriesController (/v1/preset-stories)
- DI registration; gateway route /v1/preset-stories (optionalAuth, public read)

Frontend:
- ProjectPresetStories admin authoring UI (name/description/demo upload/published/
  sort + scene picker with order+duration + advanced scenes_spa); «ویدیوهای نمونه»
  button + modal in ProjectsAdmin
- TemplateDetailExamples renders real published stories (image/video preview,
  hover → "use this example" → creates a pre-bound project), falls back to
  placeholders when none; selected aspect's variant id keys the fetch
- public /api/preset-stories route; preset_story_id plumbed through
  createProjectFromTemplate + projects POST route; usePreset i18n (fa+en)

Verified: full CRUD via gateway (public hides unpublished); creating a project
with presetStoryId persists selected_preset_story_id on the saved project.

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

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

@@ -0,0 +1,32 @@
+namespace FlatRender.ContentSvc.Models;
+
+// ── Preset stories (admin-authored premade example videos per template) ───────
+// A preset story is a ready-made, pre-filled version of a template that a user can
+// pick to start a project already populated with text + media. The filled values
+// live in ScenesSpa (the studio scene JSON); PresetScene rows order the scenes.
+
+public record PresetSceneInput(Guid SceneId, int Sort, decimal? DefaultDurationSec);
+
+public record PresetSceneResponse(
+    Guid Id, Guid SceneId, string? SceneKey, string? SceneTitle, int Sort, decimal? DefaultDurationSec
+);
+
+// Light projection for list endpoints (omits the heavy ScenesSpa blob).
+public record PresetStorySummary(
+    Guid Id, Guid ProjectId, string Name, string? Description, string? Demo,
+    Guid? MusicId, int Sort, bool IsPublished, int SceneCount,
+    DateTime CreatedAt, DateTime UpdatedAt
+);
+
+// Full story incl. scenes + the filled-values blob (GET one).
+public record PresetStoryResponse(
+    Guid Id, Guid ProjectId, string Name, string? Description, string? Demo,
+    Guid? MusicId, string? ScenesSpa, int Sort, bool IsPublished,
+    DateTime CreatedAt, DateTime UpdatedAt, List<PresetSceneResponse> Scenes
+);
+
+public record SavePresetStoryRequest(
+    Guid ProjectId, string Name, string? Description = null, string? Demo = null,
+    Guid? MusicId = null, string? ScenesSpa = null, int Sort = 0, bool IsPublished = true,
+    List<PresetSceneInput>? Scenes = null
+);