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Verified: full CRUD via gateway (public hides unpublished); creating a project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>

messages/en.json

@@ -617,7 +617,8 @@
       "mostPopular": "Most Popular"
     },
     "componentsTemplatesTemplateDetailExamples": {
-      "heading": "Videos created using this template"
+      "heading": "Videos created using this template",
+      "usePreset": "Use this example"
     },
     "componentsTemplatesTemplateDetailInfo": {
       "sceneCount": "{count} scenes",

messages/fa.json

@@ -617,7 +617,8 @@
       "mostPopular": "محبوب‌ترین"
     },
     "componentsTemplatesTemplateDetailExamples": {
-      "heading": "ویدیوهای ساخته‌شده با این قالب"
+      "heading": "ویدیوهای ساخته‌شده با این قالب",
+      "usePreset": "استفاده از این نمونه"
     },
     "componentsTemplatesTemplateDetailInfo": {
       "sceneCount": "{count} صحنه",