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/PlayCounter.cs

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+namespace DeepDrftModels.Entities;
+
+/// <summary>
+/// Incremental rollup of play counts per track (Phase 16 §4.1 / D6). One row per track, bumped inside
+/// the same transaction that appends the <see cref="PlayEvent"/> — no background aggregation job. The
+/// home card and per-target reads sum these instead of <c>COUNT(*)</c>-ing the event log on every
+/// landing. Release totals are <em>derived</em> (D4) by summing the counters of the release's tracks,
+/// so there is no separate release-counter row — this keeps the rollup normalized at one row per track.
+/// </summary>
+public class PlayCounter
+{
+    public long Id { get; set; }
+
+    /// <summary>The track these counts belong to (SQL id). Unique — one counter row per track.</summary>
+    public long TrackId { get; set; }
+
+    /// <summary>Count of plays that ended in the <c>Partial</c> bucket (&lt; 30%).</summary>
+    public long PartialCount { get; set; }
+
+    /// <summary>Count of plays that ended in the <c>Sampled</c> bucket (30%–80%).</summary>
+    public long SampledCount { get; set; }
+
+    /// <summary>Count of plays that ended in the <c>Complete</c> bucket (&gt; 80%).</summary>
+    public long CompleteCount { get; set; }
+
+    /// <summary>Total plays for the track — the sum of the three bucket counts (headline figure).</summary>
+    public long TotalPlays => PartialCount + SampledCount + CompleteCount;
+}

DeepDrftModels/Entities/PlayEvent.cs

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+using DeepDrftModels.Enums;
+
+namespace DeepDrftModels.Entities;
+
+/// <summary>
+/// Append-only log row for one recorded play (Phase 16 §4.2). Written once at session close, after the
+/// engagement floor is crossed; never updated or deleted. Deliberately NOT a <c>BaseEntity</c>: events
+/// have no soft-delete lifecycle, no <c>UpdatedAt</c> — they are immutable facts. The release link is
+/// resolved server-side from the track key at write time (§2.3 / D4) and stored here so release-total
+/// plays are a cheap sum over this column.
+/// </summary>
+public class PlayEvent
+{
+    public long Id { get; set; }
+
+    /// <summary>The played track's vault entry key (the only target the client sends).</summary>
+    public required string TrackEntryKey { get; set; }
+
+    /// <summary>
+    /// The owning release's SQL id, resolved from <see cref="TrackEntryKey"/> at write time. Null when
+    /// the track is loose (no release) or the key did not resolve to a live track at write time.
+    /// </summary>
+    public long? ReleaseId { get; set; }
+
+    /// <summary>The completion bucket computed client-side from the high-water fraction (§1a / D1).</summary>
+    public PlayBucket Bucket { get; set; }
+
+    /// <summary>
+    /// Anonymous listener token (Phase 16 §3, wave 16.3). Reserved nullable; nothing writes it in wave
+    /// 16.1 — the client sends none and the column stays NULL. Wave 16.3 lights it up for the
+    /// distinct-listener count.
+    /// </summary>
+    public string? AnonId { get; set; }
+
+    public DateTime CreatedAt { get; set; }
+}

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; }
+}