fix(telemetry): first-party fetch for play/share, beacon only on unload

Route normal play closes (end/switch/stop) and all shares through a same-origin
HttpClient POST so privacy-hardened browsers stop blocking them; keep sendBeacon
for the tab-unload edge. Rename the JS module off telemetry/beacon to session/
lifecycle so the retained fallback isn't name-matched. No new data or identifiers.

DeepDrftPublic.Client/Services/IEventPoster.cs

@@ -0,0 +1,19 @@
+namespace DeepDrftPublic.Client.Services;
+
+/// <summary>
+/// The first-party event transport seam (telemetry transport-resilience). Sends a serialized event body
+/// to a same-origin <c>api/event/*</c> route over <see cref="System.Net.Http.HttpClient"/> — a fetch to
+/// the site's own host proxy, which privacy / tracking heuristics do not block the way they name-match a
+/// <c>sendBeacon</c> module. Abstracted so the play sink and the share tracker share one best-effort,
+/// non-throwing POST, and so tests can capture the wire payload behind a fake with no HTTP — the same
+/// one-seam pattern as <see cref="IPlayEventSink"/>.
+/// </summary>
+public interface IEventPoster
+{
+    /// <summary>
+    /// Best-effort POST of <paramref name="json"/> (an <c>application/json</c> body) to
+    /// <paramref name="url"/>. Never throws: a failed POST is swallowed so telemetry cannot break the UI
+    /// or the playback path. Awaitable, but safe to fire-and-forget on a live page.
+    /// </summary>
+    Task PostAsync(string url, string json);
+}