feat(application): add OAuth2 authorization flow commands and queries (GetGoogleAuthorizationUrl, CompleteGoogleAuthorization)

src/core/DigitalData.MessagingService.Application/OAuth2/Commands/CompleteOAuth2AuthorizationCommand.cs

@@ -0,0 +1,72 @@
+#if NET
+using DigitalData.MessagingService.Application.Common.Interfaces;
+using DigitalData.MessagingService.Application.Common.Interfaces.Repositories;
+using DigitalData.MessagingService.Domain.Entities;
+using DigitalData.MessagingService.Domain.Exceptions;
+using MediatR;
+using Microsoft.Extensions.Logging;
+
+namespace DigitalData.MessagingService.Application.OAuth2.Commands;
+
+/// <summary>
+/// Exchanges a Google OAuth2 authorization code for a refresh token
+/// and persists it on the email account.
+/// Call this from the OAuth2 callback endpoint after the user grants consent.
+/// </summary>
+public record CompleteOAuth2AuthorizationCommand : IRequest<CompleteOAuth2AuthorizationResult>
+{
+    /// <summary>
+    /// ID of the email account being authorized.
+    /// </summary>
+    public required int AccountId { get; init; }
+
+    /// <summary>
+    /// The authorization code received from the OAuth2 provider callback.
+    /// </summary>
+    public required string Code { get; init; }
+
+    /// <summary>
+    /// The redirect URI used in the original authorization request.
+    /// </summary>
+    public required string RedirectUri { get; init; }
+}
+
+public record CompleteOAuth2AuthorizationResult
+{
+    public required string Username { get; init; }
+    public required bool Success { get; init; }
+    public string? ErrorMessage { get; init; }
+}
+
+public class CompleteOAuth2AuthorizationCommandHandler(
+    IRepository<EmailAccount> Repo,
+    IOAuth2AuthorizationService AuthService,
+    ILogger<CompleteOAuth2AuthorizationCommandHandler> Logger) : IRequestHandler<CompleteOAuth2AuthorizationCommand, CompleteOAuth2AuthorizationResult>
+{
+    public async Task<CompleteOAuth2AuthorizationResult> Handle(CompleteOAuth2AuthorizationCommand request, CancellationToken cancellationToken)
+    {
+        var account = await Repo.GetByIdAsync(request.AccountId, cancellationToken)
+            ?? throw new NotFoundException($"No email account found with Id: {request.AccountId}.");
+
+        Logger.LogInformation("Exchanging OAuth2 authorization code for account '{Username}' (Id: {Id}).",
+            account.Username, account.Id);
+
+        var refreshToken = await AuthService.ExchangeCodeForRefreshTokenAsync(
+            account, request.Code, request.RedirectUri, cancellationToken);
+
+        // Set the refresh token directly on the tracked entity — no AutoMapper needed
+        account.OAuth2RefreshToken = refreshToken;
+        await Repo.UpdateSingleAsync(a => a.Id == account.Id, account, cancellationToken);
+
+        Logger.LogInformation("OAuth2 refresh token saved for account '{Username}' (Id: {Id}).",
+            account.Username, account.Id);
+
+        return new CompleteOAuth2AuthorizationResult
+        {
+            Username = account.Username,
+            Success  = true
+        };
+    }
+}
+#endif
+

src/core/DigitalData.MessagingService.Application/OAuth2/Queries/GetOAuth2AuthorizationUrlQuery.cs

@@ -0,0 +1,46 @@
+#if NET
+using DigitalData.MessagingService.Application.Common.Interfaces;
+using DigitalData.MessagingService.Application.Common.Interfaces.Repositories;
+using DigitalData.MessagingService.Domain.Entities;
+using DigitalData.MessagingService.Domain.Enums;
+using DigitalData.MessagingService.Domain.Exceptions;
+using MediatR;
+
+namespace DigitalData.MessagingService.Application.OAuth2.Queries;
+
+/// <summary>
+/// Returns the authorization URL the user must visit to grant OAuth2 access.
+/// Supported for providers that require user-delegated access (e.g. Google).
+/// </summary>
+public record GetOAuth2AuthorizationUrlQuery : IRequest<string>
+{
+    /// <summary>
+    /// ID of the email account to authorize.
+    /// </summary>
+    public required int AccountId { get; init; }
+
+    /// <summary>
+    /// The redirect URI registered with the OAuth2 provider.
+    /// Must exactly match the URI configured in the provider's developer console.
+    /// </summary>
+    public required string RedirectUri { get; init; }
+}
+
+public class GetOAuth2AuthorizationUrlQueryHandler(
+    IRepository<EmailAccount> Repo,
+    IOAuth2AuthorizationService AuthService) : IRequestHandler<GetOAuth2AuthorizationUrlQuery, string>
+{
+    public async Task<string> Handle(GetOAuth2AuthorizationUrlQuery request, CancellationToken cancellationToken)
+    {
+        var account = await Repo.GetByIdAsync(request.AccountId, cancellationToken)
+            ?? throw new NotFoundException($"No email account found with Id: {request.AccountId}.");
+
+        if (account.OAuth2Provider == OAuth2Provider.None)
+            throw new BadRequestException(
+                $"Account '{account.Username}' (Id: {account.Id}) has no OAuth2 provider configured. " +
+                $"Set OAuth2Provider to a supported value (e.g. Google).");
+
+        return AuthService.GetAuthorizationUrl(account, request.RedirectUri);
+    }
+}
+#endif