feat(blog): add city_id to blogs for city-specific scoping

- Introduced a new nullable city_id column in the blogs table to allow scoping of blog posts to specific cities.
- Updated Blog entity to include a ManyToOne relationship with the City entity.
- Enhanced BlogController to handle city_id in the request, allowing filtering of posts by city.
- Modified BlogRepository to support querying published posts based on city_id.
- Added tests to ensure correct behavior for city-scoped and nationwide posts, including creation and updating of posts with city associations.

assets/admin/types/index.ts

@@ -428,6 +428,8 @@ export interface Blog {
     image_url: string | null;
     status: "draft" | "published";
     author?: { uuid: string; name: string } | null;
+    /** null = مقاله سراسری (روی همه دامنه‌ها، canonical روی دامنه اصلی) */
+    city?: { id: string; name: string } | null;
     tags: string[];
     created_at: string;
     updated_at?: string;