feat(phase-16): anonymous play & share telemetry substrate (wave 16.1)

Player-service play-session tracker (floor + 3-bucket classify), SharePopover share tracker with debounce, sendBeacon interop, proxied rate-limited POST api/event/{play,share}, append-only event logs + incremental play_counter with server-side release resolution. Migration authored, not applied. No anonId, no read surface.

DeepDrftModels/DTOs/ShareEventDto.cs

@@ -0,0 +1,19 @@
+using DeepDrftModels.Enums;
+
+namespace DeepDrftModels.DTOs;
+
+/// <summary>
+/// Wire payload for <c>POST api/event/share</c> (Phase 16 §2.2 / §4.3). The popover knows the target
+/// and channel at the point of the action, so the payload is self-describing — no server-side resolution.
+/// <see cref="AnonId"/> is reserved for wave 16.3 and stays null in wave 16.1.
+/// </summary>
+public class ShareEventDto
+{
+    public ShareTargetType TargetType { get; set; }
+
+    public string? TargetKey { get; set; }
+
+    public ShareChannel Channel { get; set; }
+
+    public string? AnonId { get; set; }
+}