Add admin CMS, EF Core data layer, site controllers, views, and services

[Code-311]

Motivation

• Introduce a simple content management area and data model to manage essays, models, tools, media, assets, and static pages.
• Provide server-side rendering for public pages and an admin UI with authentication and CRUD flows.
• Add persistence using EF Core with sensible configurations and a development in-memory database plus PostgreSQL support for production.
• Provide utilities for rendering Markdown and storing uploaded assets to support authored content.

Description

• Added a new ApplicationDbContext with entity configurations under Data/Configurations and domain entities under Models/Domain including Essay, FrameworkModel, Tool, MediaItem, UploadedAsset, StaticPage, and supporting base classes.
• Implemented site controllers (WritingsController, ToolsController, ModelsController, MediaController, PagesController, AssetsController) and admin area controllers and viewmodels under Areas/Admin with corresponding Razor views and a simple admin layout.
• Implemented services for Markdown rendering (IMarkdownRenderer / MarkdownRenderer) and asset persistence (IAssetService / AssetService), and hooked DI and Identity in Program.cs with cookie login at /account/login.
• Added DatabaseSeeder to bootstrap roles, an admin user, and initial content; updated Program.cs to register services, wire areas routing, and run the seeder; updated project file with package references and added CSS/JS assets and a small SVG logo.

Testing

• Built the project with dotnet build, which completed successfully.
• No automated unit or integration tests were added in this change set.
• The database seeder is invoked at startup via DatabaseSeeder.SeedAsync (manual/runtime verification recommended in dev environment).

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