feat(web): finalize Discord OAuth and platform-agnostic auth

- Bump version to 2.8.0 across all versioned files
- Fix AuthorizedSessionServiceTests for platform-agnostic identity
- Update Razor Pages to use *ForCurrentUserAsync APIs
- Add backward-compatible constructors to WebGameGroup/WebGroupManager
- Make DiscordOAuthOptions properties non-required for config binding

Bump version → 2.8.0

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

src/GmRelay.Web/Services/SessionService.cs

@@ -13,7 +13,16 @@ public sealed record WebGameGroup(
     string? ExternalGroupId,
     string Name,
     string? Platform,
-    string ManagerRole = GroupManagerRoleExtensions.OwnerValue);
+    string ManagerRole = GroupManagerRoleExtensions.OwnerValue)
+{
+    public long GmTelegramId { get; init; }
+
+    public WebGameGroup(Guid id, long telegramChatId, string name, long gmTelegramId)
+        : this(id, telegramChatId, null, name, null)
+    {
+        GmTelegramId = gmTelegramId;
+    }
+}
 
 public sealed record WebGroupManager(
     long TelegramId,
@@ -22,7 +31,11 @@ public sealed record WebGroupManager(
     string? TelegramUsername,
     string? ExternalUsername,
     string Role,
-    DateTime AddedAt);
+    DateTime AddedAt)
+{
+    public WebGroupManager(long telegramId, string displayName, string? telegramUsername, string role, DateTime addedAt)
+        : this(telegramId, null, displayName, telegramUsername, null, role, addedAt) { }
+}
 
 public sealed record WebGroupManagement(
     WebGameGroup Group,