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.

DeepDrftData/Repositories/EventRepository.cs

@@ -83,6 +83,44 @@ public class EventRepository
         }
     }
 
+    /// <summary>
+    /// Count distinct non-null anon ids across every play event (Phase 16 §3 / §4.2 — the all-time
+    /// unique-listener metric, D3). Null anon ids (events where the listener sent no token, or storage
+    /// was unavailable) are excluded — they are not a known listener and must not inflate the count. This
+    /// is the site-wide listener reach figure; the per-track / per-release overloads scope it.
+    /// </summary>
+    public Task<int> CountDistinctListenersAsync(CancellationToken ct = default)
+        => _context.PlayEvents
+            .Where(e => e.AnonId != null)
+            .Select(e => e.AnonId)
+            .Distinct()
+            .CountAsync(ct);
+
+    /// <summary>
+    /// Distinct listeners for one track, keyed by its vault entry key (the same key the play event
+    /// stamps). Null anon ids excluded. Per-track scope of <see cref="CountDistinctListenersAsync()"/>.
+    /// </summary>
+    public Task<int> CountDistinctListenersForTrackAsync(string trackEntryKey, CancellationToken ct = default)
+        => _context.PlayEvents
+            .Where(e => e.TrackEntryKey == trackEntryKey && e.AnonId != null)
+            .Select(e => e.AnonId)
+            .Distinct()
+            .CountAsync(ct);
+
+    /// <summary>
+    /// Distinct listeners for one release, derived across the release's tracks (D4): the play event
+    /// stamps the resolved release id at write time, so a distinct count over <c>anon_id</c> filtered by
+    /// <c>release_id</c> is exactly "distinct listeners who played any track in this release." Null anon
+    /// ids excluded. A listener who heard two tracks of the release counts once (it is a distinct count
+    /// over the union, not a sum of per-track counts).
+    /// </summary>
+    public Task<int> CountDistinctListenersForReleaseAsync(long releaseId, CancellationToken ct = default)
+        => _context.PlayEvents
+            .Where(e => e.ReleaseId == releaseId && e.AnonId != null)
+            .Select(e => e.AnonId)
+            .Distinct()
+            .CountAsync(ct);
+
     /// <summary>Append one share event. No rollup table for shares in wave 16.1 — a plain insert.</summary>
     public async Task RecordShareAsync(
         ShareTargetType targetType, string targetKey, ShareChannel channel, string? anonId,