feat(scoreboard): unit 10 — the polling policy (quiet by default)

ScoreboardCadence.Due(phase, lastAnalytics, lastSubscribers, now) is the creator's rule as a pure
function: watch hours ONLY during a live stream, every 30 minutes (Analytics answers in days and
test/record sessions have no public audience, so querying then is pure waste); subscribers whenever
the app is working, every 60 s; and nothing at all when idle.

Deliberately returns 'no poll' rather than 'analytics only' — a poll kind the poller cannot perform
is a bug waiting for a caller.

Remaining for the scoreboard: the HTTP providers behind this policy (watch hours via Analytics
reports.query, subscribers via channels.list) and the timer that calls Due(). Recorded in HANDOFF.

Models/ScoreboardCadence.cs

@@ -0,0 +1,47 @@
+using System;
+
+namespace ytLive.Models;
+
+/// <summary>What kind of session the app is in, for polling decisions.</summary>
+public enum SessionPhase
+{
+    Idle,
+    Live,
+    Test,
+    Recording,
+}
+
+/// <summary>Which channel figures a poll should fetch.</summary>
+public enum ScoreboardPoll
+{
+    None,
+    SubscribersOnly,
+    Both,
+}
+
+/// <summary>Polling policy for the scoreboard's two channel figures (creator ruling 2026-10-30).
+///
+/// <para>Analytics answers in DAYS, and test/record sessions have no public audience — so querying
+/// watch hours then is pure waste. Subscriber count is cheap (one quota unit) and moves during any
+/// session, so it polls whenever the app is working. Idle means silent.</para></summary>
+public static class ScoreboardCadence
+{
+    public static readonly TimeSpan AnalyticsInterval = TimeSpan.FromMinutes(30);
+    public static readonly TimeSpan SubscriberInterval = TimeSpan.FromSeconds(60);
+
+    public static ScoreboardPoll Due(SessionPhase phase, DateTime lastAnalyticsUtc, DateTime lastSubscriberUtc, DateTime nowUtc)
+    {
+        if (phase == SessionPhase.Idle) return ScoreboardPoll.None;
+
+        var analyticsDue = phase == SessionPhase.Live && nowUtc - lastAnalyticsUtc >= AnalyticsInterval;
+        var subscribersDue = nowUtc - lastSubscriberUtc >= SubscriberInterval;
+
+        return (analyticsDue, subscribersDue) switch
+        {
+            (true, true) => ScoreboardPoll.Both,
+            (true, false) => ScoreboardPoll.None,       // analytics alone is not a poll kind
+            (false, true) => ScoreboardPoll.SubscribersOnly,
+            _ => ScoreboardPoll.None,
+        };
+    }
+}