feat(web): add Discord OAuth service and authorization endpoints

- DiscordOAuthOptions for client_id, secret, redirect_uri
- DiscordAuthService exchanges code for token and fetches user profile
- /auth/discord and /auth/discord/callback endpoints
- CreateDiscordPrincipal for cookie auth claims
- Telegram principal now includes Platform claim for forward compatibility

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.Web/DiscordOAuthOptions.cs

@@ -0,0 +1,19 @@
+namespace GmRelay.Web;
+
+public sealed class DiscordOAuthOptions
+{
+    public required string ClientId { get; set; }
+    public required string ClientSecret { get; set; }
+    public required string RedirectUri { get; set; }
+    public string[] Scopes { get; set; } = ["identify", "guilds"];
+
+    public void Validate()
+    {
+        if (string.IsNullOrWhiteSpace(ClientId))
+            throw new InvalidOperationException("Discord:ClientId is required.");
+        if (string.IsNullOrWhiteSpace(ClientSecret))
+            throw new InvalidOperationException("Discord:ClientSecret is required.");
+        if (string.IsNullOrWhiteSpace(RedirectUri))
+            throw new InvalidOperationException("Discord:RedirectUri is required.");
+    }
+}