feat(phase-16.3): light up anonId unique-listener layer

Mint a first-party localStorage anonId, thread it onto play/share beacons,
persist it via EventController, and add all-time distinct-listener counts
(site/track/release). Storage columns + indexes already existed from 16.1.

DeepDrftPublic.Client/Services/BeaconPlayEventSink.cs

@@ -1,4 +1,5 @@
 using System.Text.Json;
+using System.Text.Json.Serialization;
 using DeepDrftModels.DTOs;
 using DeepDrftModels.Enums;
 using Microsoft.AspNetCore.Components;
@@ -10,16 +11,26 @@ namespace DeepDrftPublic.Client.Services;
 /// it via <c>navigator.sendBeacon</c> to the proxied <c>api/event/play</c> route. Fire-and-forget by
 /// design — <see cref="IPlayEventSink.EmitPlay"/> is synchronous (it is called from the player's close
 /// path and the unload handler, neither of which can await), so the beacon is dispatched without
-/// awaiting and its failure is irrelevant. No <c>anonId</c> is sent in wave 16.1.
+/// awaiting and its failure is irrelevant. The current <c>anonId</c> (wave 16.3) is read synchronously
+/// from the warmed <see cref="IAnonIdProvider"/> cache and omitted when null (storage unavailable / not
+/// yet warmed) — an anonId-less play still counts, it just doesn't contribute to the listener tally.
 /// </summary>
 public sealed class BeaconPlayEventSink : IPlayEventSink
 {
+    // Omit a null anonId from the wire payload (§2.2 — "omitted entirely" when absent) rather than
+    // sending "anonId":null. The API treats absent and null identically, so this is cosmetic minimalism;
+    // it does not change the integer enum encoding the 16.1 contract already relies on.
+    private static readonly JsonSerializerOptions BeaconJson =
+        new() { DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull };
+
     private readonly BeaconInterop _beacon;
+    private readonly IAnonIdProvider _anonId;
     private readonly string _playUrl;
 
-    public BeaconPlayEventSink(BeaconInterop beacon, NavigationManager navigation)
+    public BeaconPlayEventSink(BeaconInterop beacon, IAnonIdProvider anonId, NavigationManager navigation)
     {
         _beacon = beacon;
+        _anonId = anonId;
         // The WASM client posts to its own host, which proxies to DeepDrftAPI. BaseUri carries a
         // trailing slash; the route does not lead with one.
         _playUrl = $"{navigation.BaseUri}api/event/play";
@@ -31,7 +42,8 @@ public sealed class BeaconPlayEventSink : IPlayEventSink
         {
             TrackEntryKey = trackEntryKey,
             Bucket = bucket,
-        });
+            AnonId = _anonId.Current,
+        }, BeaconJson);
 
         // Fire-and-forget: do not await. The beacon survives unload; the C# task may not, and we do not
         // act on the result either way.