🏗️feat: remove frontend to adopt a pure headless API architecture

- Removed the Blazor Server frontend to refocus the project as a pure
backend API, designed to serve multiple clients (web, mobile, external).
- Cleaned up Program.cs by removing Razor Pages, static files and
frontend-specific CORS policies.
- Optimized Docker build and .dockerignore to include only backend
projects, reducing image size, build time and attack surface.

Author: alysonsz