feat(web): dashboard session deletion and E2E coverage for issue #150

- Add DeleteSessionAsync to ISessionStore/SessionService (unpublish portfolio card,
  remove bot-created empty forum topic, update batch message).
- Add DeleteSessionForCurrentUserAsync to AuthorizedSessionService with audit log.
- Add delete button + confirmation dialog to GroupDetails.razor.
- Extend dashboard Playwright tests with edit persistence and delete verification.
- Update AuthorizedSessionServiceTests with delete authorization coverage.
- Mark issue #150 as done in tests/e2e/README.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/GmRelay.Web/Services/SessionService.cs

@@ -129,6 +129,14 @@ internal sealed record WebBatchSessionRow(
     string? CoverImageUrl = null);
 internal sealed record WebTemplateGroupDto(long TelegramChatId);
 internal sealed record WebTemplateTopicDestination(int? MessageThreadId, bool TopicCreatedByBot);
+internal sealed record WebDeleteSessionInfo(
+    Guid Id,
+    string Title,
+    Guid BatchId,
+    long TelegramChatId,
+    int? BatchMessageId,
+    int? ThreadId,
+    bool TopicCreatedByBot);
 internal sealed record WebPublicGroupRow(
     Guid GroupId,
     string Name,
@@ -1086,6 +1094,95 @@ public sealed class SessionService(
         }
     }
 
+    public async Task<string?> DeleteSessionAsync(Guid sessionId, Guid groupId)
+    {
+        await using var conn = await dataSource.OpenConnectionAsync();
+        await using var transaction = await conn.BeginTransactionAsync();
+
+        var session = await conn.QuerySingleOrDefaultAsync<WebDeleteSessionInfo>(
+            """
+            SELECT s.id AS Id,
+                   s.title AS Title,
+                   s.batch_id AS BatchId,
+                   g.external_group_id::BIGINT AS TelegramChatId,
+                   s.batch_message_id AS BatchMessageId,
+                   s.thread_id AS ThreadId,
+                   s.topic_created_by_bot AS TopicCreatedByBot
+            FROM sessions s
+            JOIN game_groups g ON g.id = s.group_id
+            WHERE s.id = @SessionId AND s.group_id = @GroupId
+            FOR UPDATE
+            """,
+            new { SessionId = sessionId, GroupId = groupId },
+            transaction);
+
+        if (session is null)
+        {
+            throw new SessionAccessDeniedException(sessionId, "0");
+        }
+
+        await conn.ExecuteAsync(
+            """
+            UPDATE portfolio_games pg
+            SET is_public = false,
+                updated_at = now()
+            FROM portfolio_game_sessions pgs
+            WHERE pgs.portfolio_game_id = pg.id
+              AND pgs.session_id = @SessionId
+              AND pg.is_public = true
+            """,
+            new { SessionId = sessionId },
+            transaction);
+
+        await conn.ExecuteAsync(
+            "DELETE FROM sessions WHERE id = @Id AND group_id = @GroupId",
+            new { Id = sessionId, GroupId = groupId },
+            transaction);
+
+        var remainingInTopic = session.ThreadId.HasValue
+            ? await conn.ExecuteScalarAsync<int>(
+                """
+                SELECT COUNT(*)
+                FROM sessions
+                WHERE group_id = @GroupId
+                  AND thread_id = @ThreadId
+                """,
+                new { GroupId = groupId, ThreadId = session.ThreadId.Value },
+                transaction)
+            : 0;
+
+        await transaction.CommitAsync();
+
+        if (session.ThreadId.HasValue && session.TopicCreatedByBot && remainingInTopic == 0)
+        {
+            try
+            {
+                await bot.DeleteForumTopic(
+                    chatId: session.TelegramChatId,
+                    messageThreadId: session.ThreadId.Value);
+                logger.LogInformation(
+                    "Deleted forum topic {ThreadId} for group {GroupId} as no sessions remained.",
+                    session.ThreadId.Value,
+                    groupId);
+            }
+            catch (Exception ex)
+            {
+                logger.LogWarning(
+                    ex,
+                    "Failed to delete forum topic {ThreadId} for group {GroupId}",
+                    session.ThreadId.Value,
+                    groupId);
+            }
+        }
+
+        if (session.BatchMessageId.HasValue)
+        {
+            await TryUpdateBatchMessageAsync(session.BatchId, session.TelegramChatId, session.BatchMessageId.Value, session.Title);
+        }
+
+        return session.Title;
+    }
+
     public async Task PromoteWaitlistedPlayerAsync(Guid sessionId, Guid groupId)
     {
         await using var conn = await dataSource.OpenConnectionAsync();