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.

Services/YppTrackerViewModel.cs

@@ -1,4 +1,7 @@
+using System;
+using System.Collections.Generic;
 using System.Diagnostics;
+using System.Linq;
 using System.Windows.Input;
 using ytLive.Helpers;
 using ytLive.Models;
@@ -25,6 +28,12 @@ public sealed class YppTrackerViewModel : ViewModelBase
     private readonly Func<bool> _isConnected;
     private readonly Func<string?> _displayName;
 
+    // TASK 3·20 — the session report is DERIVED, never stored: ask YouTube for the channel's recent
+    // reward events and roll them up for the window that starts when this session did.
+    private readonly Func<ISuperChatEventSource> _rewardEvents;
+    private readonly Func<string?> _channelId;
+    private readonly Func<DateTime?> _sessionStartedAtUtc;
+
     private bool _isDrawerOpen;
     private bool _isLoading;
     private bool _isRefreshing;
@@ -43,12 +52,18 @@ public sealed class YppTrackerViewModel : ViewModelBase
         Func<LayoutStore> store,
         Func<ChannelStatsService> stats,
         Func<bool> isConnected,
-        Func<string?> displayName)
+        Func<string?> displayName,
+        Func<ISuperChatEventSource>? rewardEvents = null,
+        Func<string?>? channelId = null,
+        Func<DateTime?>? sessionStartedAtUtc = null)
     {
         _store = store;
         _stats = stats;
         _isConnected = isConnected;
         _displayName = displayName;
+        _rewardEvents = rewardEvents ?? (() => null!);
+        _channelId = channelId ?? (() => null);
+        _sessionStartedAtUtc = sessionStartedAtUtc ?? (() => null);
 
         RefreshCommand = new RelayCommand(_ => _ = RefreshAsync());
         ToggleDrawerCommand = new RelayCommand(_ => IsDrawerOpen = !IsDrawerOpen);
@@ -58,6 +73,7 @@ public sealed class YppTrackerViewModel : ViewModelBase
         OpenAdsenseCommand = new RelayCommand(_ => OpenUrl(AdsenseGuideUrl));
 
         (_twoFactorEnabled, _adsenseLinked) = _store().LoadYppChecklist();
+        _showMonetaryMetrics = _store().LoadShowMonetaryMetrics();
         _lastRefreshedAtUtc = _store().LoadYppLastRefreshedAtUtc();
     }
 
@@ -92,6 +108,63 @@ public sealed class YppTrackerViewModel : ViewModelBase
 
     public bool IsSignedOut => !_isConnected();
 
+    private SessionRewardSummary _sessionRewards = SessionRewardSummary.Empty;
+
+    /// <summary>This session's reward events, derived on demand. Never persisted.</summary>
+    public SessionRewardSummary SessionRewards
+    {
+        get => _sessionRewards;
+        private set
+        {
+            _sessionRewards = value;
+            OnPropertyChanged(nameof(SessionRewards));
+            OnPropertyChanged(nameof(HasSessionRewards));
+            OnPropertyChanged(nameof(SessionCountsText));
+            OnPropertyChanged(nameof(SessionMoneyLines));
+            OnPropertyChanged(nameof(HasSessionMoney));
+        }
+    }
+
+    public bool HasSessionRewards => _sessionRewards.HasEvents;
+
+    /// <summary>Counts are always neutral — "3 Super Chats" says nothing about money, and most
+    /// creators have none.</summary>
+    public string SessionCountsText
+    {
+        get
+        {
+            if (!_sessionRewards.HasEvents) return string.Empty;
+            var parts = new List<string>();
+            if (_sessionRewards.SuperChatCount > 0) parts.Add($"{_sessionRewards.SuperChatCount} Super Chat{(_sessionRewards.SuperChatCount == 1 ? "" : "s")}");
+            if (_sessionRewards.SuperStickerCount > 0) parts.Add($"{_sessionRewards.SuperStickerCount} Super Sticker{(_sessionRewards.SuperStickerCount == 1 ? "" : "s")}");
+            return string.Join(" · ", parts);
+        }
+    }
+
+    /// <summary>Money is shown only when the creator asked for it AND there is something to show.
+    /// A creator who never monetized never sees a zero (creator ruling 2026-11-05).</summary>
+    public IReadOnlyList<string> SessionMoneyLines
+        => ShowMonetaryMetrics ? _sessionRewards.MoneyLines() : Array.Empty<string>();
+
+    public bool HasSessionMoney => SessionMoneyLines.Count > 0;
+
+    private bool _showMonetaryMetrics;
+
+    /// <summary>"Show monetary metrics" — lives in this drawer, persisted, OFF by default. Turning it on
+    /// is also what would justify requesting the monetary Analytics scope; while it is off the app never
+    /// asks Google for money data.</summary>
+    public bool ShowMonetaryMetrics
+    {
+        get => _showMonetaryMetrics;
+        set
+        {
+            if (!SetProperty(ref _showMonetaryMetrics, value)) return;
+            _store().SaveShowMonetaryMetrics(value);
+            OnPropertyChanged(nameof(SessionMoneyLines));
+            OnPropertyChanged(nameof(HasSessionMoney));
+        }
+    }
+
     public string? LoadError
     {
         get => _loadError;
@@ -203,11 +276,38 @@ public sealed class YppTrackerViewModel : ViewModelBase
         await RefreshAsync();
     }
 
+    /// <summary>Asks YouTube for the channel's recent reward events and keeps only this session's.
+    /// Nothing is written down: the report is a view over YouTube's data, not a local ledger.</summary>
+    private async Task RefreshSessionRewardsAsync()
+    {
+        var started = _sessionStartedAtUtc();
+        var channel = _channelId();
+        var source = _rewardEvents();
+        if (started == null || string.IsNullOrWhiteSpace(channel) || source == null)
+        {
+            SessionRewards = SessionRewardSummary.Empty;
+            return;
+        }
+
+        try
+        {
+            var events = await source.GetRecentAsync(channel);
+            SessionRewards = SessionRewardSummary.From(events, started.Value);
+        }
+        catch (Exception ex)
+        {
+            AppLog.Write($"Session rewards unavailable: {ex.Message}");
+            SessionRewards = SessionRewardSummary.Empty;
+        }
+    }
+
     public async Task RefreshAsync()
     {
         if (_isRefreshing) return;
         _isRefreshing = true;
         IsLoading = true;
+
+        await RefreshSessionRewardsAsync();
         LoadError = null;
         try
         {