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/Entities/ShareEvent.cs

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+using DeepDrftModels.Enums;
+
+namespace DeepDrftModels.Entities;
+
+/// <summary>
+/// Append-only log row for one recorded share (Phase 16 §4.2). Written once per share action that
+/// survives the per-(target,channel) client debounce; never updated or deleted. Like <see cref="PlayEvent"/>
+/// it is deliberately NOT a <c>BaseEntity</c> — an immutable fact with no soft-delete lifecycle. Shares
+/// carry their target directly (the popover knows track vs. release), so no server-side resolution step.
+/// </summary>
+public class ShareEvent
+{
+    public long Id { get; set; }
+
+    /// <summary>Whether the share targets a track or a release.</summary>
+    public ShareTargetType TargetType { get; set; }
+
+    /// <summary>
+    /// The shared target's key: a track's vault <c>EntryKey</c> or a release's public <c>EntryKey</c>,
+    /// selected by <see cref="TargetType"/>. Stored as the opaque key, not resolved to a SQL id — the
+    /// share metric is a simple per-target tally and needs no join in wave 16.1.
+    /// </summary>
+    public required string TargetKey { get; set; }
+
+    /// <summary>The channel the share was performed through (link vs. embed).</summary>
+    public ShareChannel Channel { get; set; }
+
+    /// <summary>
+    /// Anonymous listener token (Phase 16 §3, wave 16.3). Reserved nullable; unused in wave 16.1.
+    /// </summary>
+    public string? AnonId { get; set; }
+
+    public DateTime CreatedAt { get; set; }
+}