Implement POST /posts endpoint with validation (Backend)Feature/tam create post

[tamnguyen976]

Summary

• Added POST /posts endpoint in PostsController
• Added server-side validation: returns 400 if title is blank
• Uses in-memory list for dev mode (no database required)
• Returns 201 Created with the new post's slug

Testing Steps

1. Run backend:
   dotnet run --project MyFirstBlog
2. In another terminal, create post:
   curl -X POST http://localhost:5000/posts
   -H "Content-Type: application/json"
   -d '{"title":"Hello","description":"World"}'
3. Expected: 201 Created, JSON of new post
4. Test with blank title:
   curl -X POST http://localhost:5000/posts
   -H "Content-Type: application/json"
   -d '{"title":"","description":"World"}'
   Expected: 400 Bad Request, "Title cannot be blank"

Notes

• This PR completes backend part of assignment step 3.