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>

tests/GmRelay.Bot.Tests/Web/AuthorizedSessionServiceTests.cs

@@ -765,6 +765,58 @@ public sealed class AuthorizedSessionServiceTests
         Assert.False(store.CreateBatchFromTemplateCalled);
     }
 
+    [Fact]
+    public async Task DeleteSessionForCurrentUserAsync_Deletes_WhenSessionBelongsToManager()
+    {
+        var gmId = 1001L;
+        var groupId = Guid.NewGuid();
+        var sessionId = Guid.NewGuid();
+        var store = new FakeSessionStore(
+            groups:
+            [
+                new(groupId, 42, "Alpha", gmId)
+            ],
+            sessions:
+            [
+                new(sessionId, groupId, "Session A", DateTime.UtcNow, "Planned", "https://example.test/a", Guid.NewGuid(), 10, 42, 4, 1, 0)
+            ]);
+        var accessor = CreateAccessor(gmId.ToString(System.Globalization.CultureInfo.InvariantCulture));
+        var service = new AuthorizedSessionService(store, accessor);
+
+        await service.DeleteSessionForCurrentUserAsync(sessionId);
+
+        Assert.True(store.DeleteCalled);
+        Assert.Equal(sessionId, store.LastDeletedSessionId);
+        Assert.Equal(groupId, store.LastDeletedGroupId);
+        Assert.Null(await store.GetSessionAsync(sessionId));
+    }
+
+    [Fact]
+    public async Task DeleteSessionForCurrentUserAsync_Throws_WhenSessionDoesNotBelongToManager()
+    {
+        var gmId = 1001L;
+        var otherGmId = 2002L;
+        var groupId = Guid.NewGuid();
+        var otherGroupId = Guid.NewGuid();
+        var sessionId = Guid.NewGuid();
+        var store = new FakeSessionStore(
+            groups:
+            [
+                new(groupId, 42, "Alpha", gmId),
+                new(otherGroupId, 43, "Beta", otherGmId)
+            ],
+            sessions:
+            [
+                new(sessionId, otherGroupId, "Session B", DateTime.UtcNow, "Planned", "https://example.test/b", Guid.NewGuid(), 10, 43, 4, 1, 0)
+            ]);
+        var accessor = CreateAccessor(gmId.ToString(System.Globalization.CultureInfo.InvariantCulture));
+        var service = new AuthorizedSessionService(store, accessor);
+
+        await Assert.ThrowsAsync<SessionAccessDeniedException>(
+            () => service.DeleteSessionForCurrentUserAsync(sessionId));
+        Assert.False(store.DeleteCalled);
+    }
+
     private sealed class FakeSessionStore(
         IEnumerable<WebGameGroup>? groups = null,
         IEnumerable<WebSession>? sessions = null,
@@ -796,6 +848,9 @@ public sealed class AuthorizedSessionServiceTests
         public DateTime? LastUpdatedScheduledAt { get; private set; }
         public string? LastUpdatedJoinLink { get; private set; }
         public int? LastUpdatedMaxPlayers { get; private set; }
+        public bool DeleteCalled { get; private set; }
+        public Guid? LastDeletedSessionId { get; private set; }
+        public Guid? LastDeletedGroupId { get; private set; }
         public Guid? LastPromotedSessionId { get; private set; }
         public Guid? LastPromotedGroupId { get; private set; }
         public Guid? LastUpdatedBatchId { get; private set; }
@@ -1036,6 +1091,15 @@ public sealed class AuthorizedSessionServiceTests
             return Task.CompletedTask;
         }
 
+        public Task<string?> DeleteSessionAsync(Guid sessionId, Guid groupId)
+        {
+            DeleteCalled = true;
+            LastDeletedSessionId = sessionId;
+            LastDeletedGroupId = groupId;
+            sessionsById.Remove(sessionId);
+            return Task.FromResult<string?>("Deleted session");
+        }
+
         public Task PromoteWaitlistedPlayerAsync(Guid sessionId, Guid groupId)
         {
             PromoteCalled = true;