Introduce CMS-backed content models, admin area, services, and DB seeding

[Code-311]

Motivation

• Replace hard-coded homepage content with a database-backed content model and provide an admin UI for creating and managing essays, models, tools, media, assets, and static pages.
• Add authentication and role-based authorization so site content can be managed by administrators and uploaded assets can be served securely.
• Provide Markdown rendering and an asset service to store and serve uploaded media from the database.
• Add initial seed data to bootstrap the site for development and production environments.

Description

• Added Entity Framework Core ApplicationDbContext, entity models under Models/Domain, and EF configuration classes in Data/Configurations to model publishable content like Essay, FrameworkModel, Tool, MediaItem, UploadedAsset, StaticPage, ToolQuestion, and SiteSetting.
• Implemented admin area controllers and view models under Areas/Admin for CRUD of content (EssaysController, FrameworkModelsController, ToolsController, ToolQuestionsController, MediaItemsController, StaticPagesController, UploadedAssetsController) and added basic Razor views and a shared admin layout.
• Added public controllers and views to serve content from the database (WritingsController, ModelsController, ToolsController, MediaController, PagesController, AssetsController), plus AccountController and a login view to support sign-in via Identity.
• Introduced services IMarkdownRenderer/MarkdownRenderer and IAssetService/AssetService to render Markdown and persist uploaded files, and registered them along with Identity and ApplicationDbContext changes in Program.cs (including in-memory DB for development and PostgreSQL for production).
• Added DatabaseSeeder to create the Admin role, an admin user (password read from configuration), and seed initial essays, models, tools, media, pages, and questions; added appsettings.json and appsettings.Development.json for configuration and updated project references in manpreetsingh.pro.csproj for Markdig, EF Core, Identity, and Npgsql.
• Reworked front-end assets: new site CSS, JS for theme toggling and localStorage, a logo SVG, and updated Razor layout and public views to consume the new domain models and view models.

Testing

• No automated tests were added or executed as part of this change; there are currently no unit or integration tests included in the patch.

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