Add ASP.NET Core Identity with OIDC/OAuth2 support

[wendellmva]

Summary

Add proper authentication with ASP.NET Core Identity and OIDC/OAuth2 support while maintaining backward compatibility with API key authentication.

Current State

• Simple API key authentication only (X-NuGet-ApiKey header)
• Single global API key from configuration
• No user accounts or roles

Target State

• ASP.NET Core Identity for user management
• OIDC/OAuth2 for SSO integration
• Per-user API keys stored in database
• Backward compatible with existing API key auth

New Packages

• Microsoft.AspNetCore.Identity.EntityFrameworkCore 9.0.0
• Microsoft.AspNetCore.Authentication.OpenIdConnect 9.0.0

Tasks

• Add Identity packages to BaGet.Core.csproj and BaGet.csproj
• Create BaGetUser entity (extends IdentityUser)
• Create UserApiKey entity for per-user API keys
• Update AbstractContext to inherit from IdentityDbContext<BaGetUser>
• Configure Identity in Startup.cs
• Configure OIDC authentication
• Create CompositeAuthenticationService (supports multiple auth methods)
• Add Login/Logout Razor Pages
• Add API Key management page
• Generate database migration for Identity tables
• Update authorization on controllers

New Entities

public class BaGetUser : IdentityUser
{
    public ICollection<UserApiKey> ApiKeys { get; set; }
}

public class UserApiKey
{
    public int Id { get; set; }
    public string Key { get; set; }
    public string Description { get; set; }
    public DateTime Created { get; set; }
    public DateTime? Expires { get; set; }
    public string UserId { get; set; }
    public BaGetUser User { get; set; }
}

Configuration

{
  "Authentication": {
    "Type": "None",
    "ApiKey": "",
    "OIDC": {
      "Authority": "",
      "ClientId": "",
      "ClientSecret": ""
    }
  }
}

Files to Modify

• src/BaGet.Core/BaGet.Core.csproj
• src/BaGet/BaGet.csproj
• src/BaGet.Core/Entities/AbstractContext.cs
• src/BaGet/Startup.cs
• src/BaGet/appsettings.json
• src/BaGet.Core/Authentication/ (new files)
• src/BaGet.Web/Pages/Account/ (new pages)

Dependencies

• Depends on .NET 9 upgrade
• Depends on PostgreSQL migration