feat(backend): add RequestDiscordTokenAsync method

Foxnouns.Backend/Services/RemoteAuthService.cs

@@ -0,0 +1,48 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Web;
+
+namespace Foxnouns.Backend.Services;
+
+public class RemoteAuthService(Config config)
+{
+    private readonly HttpClient _httpClient = new();
+
+    private readonly Uri _discordTokenUri = new("https://discord.com/api/oauth2/token");
+    private readonly Uri _discordUserUri = new("https://discord.com/api/v10/users/@me");
+
+    public async Task<RemoteUser> RequestDiscordTokenAsync(string code, string state)
+    {
+        var redirectUri = $"{config.BaseUrl}/auth/login/discord";
+        var resp = await _httpClient.PostAsync(_discordTokenUri, new FormUrlEncodedContent(
+            new Dictionary<string, string>
+            {
+                { "client_id", config.DiscordAuth.ClientId! },
+                { "client_secret", config.DiscordAuth.ClientSecret! },
+                { "grant_type", "authorization_code" },
+                { "code", code },
+                { "redirect_uri", redirectUri }
+            }
+        ));
+        resp.EnsureSuccessStatusCode();
+        var token = await resp.Content.ReadFromJsonAsync<DiscordTokenResponse>();
+        if (token == null) throw new FoxnounsError("Discord token response was null");
+
+        var req = new HttpRequestMessage(HttpMethod.Get, _discordUserUri);
+        req.Headers.Add("Authorization", $"{token.token_type} {token.access_token}");
+
+        var resp2 = await _httpClient.SendAsync(req);
+        resp2.EnsureSuccessStatusCode();
+        var user = await resp2.Content.ReadFromJsonAsync<DiscordUserResponse>();
+        if (user == null) throw new FoxnounsError("Discord user response was null");
+
+        return new RemoteUser(user.id, user.username);
+    }
+
+    [SuppressMessage("ReSharper", "InconsistentNaming")]
+    private record DiscordTokenResponse(string access_token, string token_type);
+
+    [SuppressMessage("ReSharper", "InconsistentNaming")]
+    private record DiscordUserResponse(string id, string username);
+
+    public record RemoteUser(string Id, string Username);
+}