feat(ypp): session report derived from YouTube — no ledger, money off by default (TASK 3·20)

The task asked for a `RewardEvents` table plus a backfill. That would have contradicted the published
promise that no viewer, member or donor information is retained after the session ends, and it would
have meant owning a ledger that YouTube already keeps.

So the report is DERIVED on demand instead:

- `Services/SuperChatEventService.cs` — `superChatEvents.list`, parsed for both event types (a sticker
  must not be dropped), authorised by the existing `auth/youtube` scope, so no re-consent.
- `Models/SessionRewardSummary.cs` — rolls events up for the session window with **per-currency totals
  that are never summed together** (a $5 and a €5 are not $10). Nothing is written down.
- The YPP drawer gains a **THIS SESSION** section: counts are neutral and always shown when there were
  events; money appears only when the creator asks for it. `Show monetary metrics` is a drawer toggle,
  persisted, **off by default** — a creator who never monetized is never shown a $0.00.

Also: money is labelled as estimates (removed/refunded events are excluded and totals can be restated
for weeks, not days).

Tests: `SessionRewardReportTests` — both reward types parse, the window makes it a session rather than
a channel report, currencies stay separate, an empty session shows nothing, a sticker-only session
still counts. `BackOnAirTests` moves into the serialised `MainViewModelLayoutPath` collection: it builds
a MainViewModel directly and was flaking against parallel tests that swap the static layout path.

FAST gate: 557 passed, 0 failed, 0 warnings.

ViewModels/MainViewModel.Streaming.Operations.cs

@@ -120,6 +120,9 @@ public partial class MainViewModel : ViewModelBase
             ? "LlamaCasty"
             : $"{StreamTitle} — LlamaCasty";
         StreamStatus = StreamStatus.Streaming;
+        // TASK 3·20: the session report window starts here. Derived on demand from YouTube, never
+        // persisted, and reset when the session ends.
+        _sessionStartedAtUtc = DateTime.UtcNow;
         ResetHealth(StreamStatus.Streaming);
         _audioMixer.StartLive(EncoderOptions.DefaultAudioPipeName);
         _liveElapsed = TimeSpan.Zero;
@@ -371,6 +374,7 @@ public partial class MainViewModel : ViewModelBase
         var wasRecording = IsRecording;
 
         StreamStatus = StreamStatus.Offline;
+        _sessionStartedAtUtc = null;
         IsRecording = false;
         LiveScene = null;
         EditingActive = true;

ViewModels/MainViewModel.Streaming.cs

@@ -416,6 +416,10 @@ public partial class MainViewModel : ViewModelBase
         set => SetProperty(ref _previewGlowThickness, value);
     }
 
+    /// <summary>When the current/last streaming session started — the window a session report covers.
+    /// Null when no session has run since launch.</summary>
+    private DateTime? _sessionStartedAtUtc;
+
     public string LiveElapsedText
     {
         get => _liveElapsedText;

ViewModels/MainViewModel.Ypp.cs

@@ -23,7 +23,10 @@ public partial class MainViewModel
             () => _layoutStore,
             () => ChannelStatsFactoryOverride?.Invoke() ?? new ChannelStatsService(_youtubeAuth),
             () => IsConnected,
-            () => _youtubeAuth.CurrentChannel?.DisplayName);
+            () => _youtubeAuth.CurrentChannel?.DisplayName,
+            rewardEvents: () => new SuperChatEventService(_youtubeAuth),
+            channelId: () => _youtubeAuth.CurrentChannel?.ChannelId,
+            sessionStartedAtUtc: () => _sessionStartedAtUtc);
 
         TestSession = new TestSessionViewModel(
             _youtubeStream,