feat(web): add platform-agnostic identity extraction from ClaimsPrincipal

- TryGetPlatformIdentity returns (platform, external_user_id)
- TryGetDiscordId for Discord-specific flows
- Backward-compatible fallback for legacy Telegram auth without Platform claim
- GetAvatarUrl helper for Discord avatars

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

src/GmRelay.Web/Services/ClaimsPrincipalExtensions.cs

@@ -6,4 +6,37 @@ public static class ClaimsPrincipalExtensions
 {
     public static bool TryGetTelegramId(this ClaimsPrincipal user, out long telegramId) =>
         long.TryParse(user.FindFirst("TelegramId")?.Value, out telegramId);
+
+    public static bool TryGetDiscordId(this ClaimsPrincipal user, out string? discordId)
+    {
+        discordId = user.FindFirst("DiscordId")?.Value;
+        return !string.IsNullOrWhiteSpace(discordId);
+    }
+
+    public static bool TryGetPlatformIdentity(this ClaimsPrincipal user, out string platform, out string externalUserId)
+    {
+        platform = string.Empty;
+        externalUserId = string.Empty;
+
+        var platformClaim = user.FindFirst("Platform")?.Value;
+        if (!string.IsNullOrWhiteSpace(platformClaim))
+        {
+            platform = platformClaim;
+            externalUserId = user.FindFirst(ClaimTypes.NameIdentifier)?.Value ?? string.Empty;
+            return !string.IsNullOrWhiteSpace(externalUserId);
+        }
+
+        // Fallback for legacy Telegram users before Platform claim was added
+        if (TryGetTelegramId(user, out var telegramId))
+        {
+            platform = "Telegram";
+            externalUserId = telegramId.ToString(System.Globalization.CultureInfo.InvariantCulture);
+            return true;
+        }
+
+        return false;
+    }
+
+    public static string? GetAvatarUrl(this ClaimsPrincipal user) =>
+        user.FindFirst("AvatarUrl")?.Value;
 }