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.

ytLive.Tests/SessionRewardReportTests.cs

@@ -0,0 +1,104 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Xunit;
+using ytLive.Models;
+using ytLive.Services;
+
+namespace ytLive.Tests;
+
+/// <summary>
+/// TASK 3·20 — the session report is DERIVED from YouTube, never stored: no ledger, no migration, and the
+/// published promise ("no viewer, member or donor information is retained after the session ends") stays
+/// literally true.
+/// <para>Two rules the creator set (2026-11-05): money is hidden unless asked for, and a creator who never
+/// monetized is never shown a zero. Counts stay neutral so a normal session still has something to read.</para>
+/// </summary>
+public class SessionRewardReportTests
+{
+    private static readonly DateTime SessionStart = new(2026, 11, 5, 20, 0, 0, DateTimeKind.Utc);
+
+    [Fact]
+    public void ParsesBothRewardTypes_WithoutDroppingStickers()
+    {
+        const string json = """
+        {
+          "items": [
+            {"id":"sc1","snippet":{"publishedAt":"2026-11-05T20:05:00Z","superChatDetails":
+              {"amountMicros":5000000,"currency":"USD","amountDisplayString":"$5.00","userComment":"hi"}}},
+            {"id":"ss1","snippet":{"publishedAt":"2026-11-05T20:07:00Z","superStickerDetails":
+              {"amountMicros":2000000,"currency":"EUR","amountDisplayString":"€2.00"}}}
+          ]
+        }
+        """;
+
+        var events = SuperChatEventService.Parse(json);
+
+        Assert.Equal(2, events.Count);
+        Assert.Equal(ChatEventKind.SuperChat, events[0].Kind);
+        Assert.Equal(5_000_000, events[0].AmountMicros);
+        Assert.Equal("USD", events[0].Currency);
+        Assert.Equal(ChatEventKind.SuperSticker, events[1].Kind);
+        Assert.Equal("EUR", events[1].Currency);
+    }
+
+    [Fact]
+    public void TheWindowMakesItASessionReport_NotAChannelReport()
+    {
+        var events = new[]
+        {
+            Event(ChatEventKind.SuperChat, 5_000_000, "USD", SessionStart.AddMinutes(-30)),  // before
+            Event(ChatEventKind.SuperChat, 3_000_000, "USD", SessionStart.AddMinutes(10)),
+            Event(ChatEventKind.SuperSticker, 2_000_000, "USD", SessionStart.AddMinutes(20)),
+        };
+
+        var summary = SessionRewardSummary.From(events, SessionStart);
+
+        Assert.Equal(1, summary.SuperChatCount);
+        Assert.Equal(1, summary.SuperStickerCount);
+        Assert.Equal(5_000_000, summary.MicrosByCurrency["USD"]);
+    }
+
+    [Fact]
+    public void CurrenciesAreNeverSummedTogether()
+    {
+        var events = new[]
+        {
+            Event(ChatEventKind.SuperChat, 5_000_000, "USD", SessionStart.AddMinutes(1)),
+            Event(ChatEventKind.SuperChat, 5_000_000, "EUR", SessionStart.AddMinutes(2)),
+        };
+
+        var summary = SessionRewardSummary.From(events, SessionStart);
+
+        Assert.Equal(5_000_000, summary.MicrosByCurrency["USD"]);
+        Assert.Equal(5_000_000, summary.MicrosByCurrency["EUR"]);
+        Assert.Equal(2, summary.MoneyLines().Count);
+        Assert.Contains(summary.MoneyLines(), line => line.StartsWith("USD $5.00"));
+        Assert.Contains(summary.MoneyLines(), line => line.StartsWith("EUR €5.00"));
+    }
+
+    [Fact]
+    public void AnEmptySession_HasNoCountsAndNoMoney()
+    {
+        var summary = SessionRewardSummary.From(Array.Empty<SuperChatEvent>(), SessionStart);
+
+        Assert.False(summary.HasEvents);
+        Assert.Empty(summary.MoneyLines());
+        Assert.Equal(0, summary.TotalEvents);
+    }
+
+    [Fact]
+    public void AStickerOnlySession_StillCounts()
+    {
+        var summary = SessionRewardSummary.From(
+            new[] { Event(ChatEventKind.SuperSticker, 1_000_000, "USD", SessionStart.AddMinutes(1)) },
+            SessionStart);
+
+        Assert.True(summary.HasEvents);
+        Assert.Equal(0, summary.SuperChatCount);
+        Assert.Equal(1, summary.SuperStickerCount);
+    }
+
+    private static SuperChatEvent Event(ChatEventKind kind, long micros, string currency, DateTime at)
+        => new($"id-{Guid.NewGuid():N}", kind, micros, currency, null, at);
+}