feat(missions): add 3 viewer entities + Viewer collections

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SVSim.Database/Models/ViewerEventCounter.cs

@@ -0,0 +1,15 @@
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// Per-viewer "how many times has this happened" counter. Composite PK
+/// (ViewerId, EventKey, Period). Period strings: "all-time", "month:YYYY-MM",
+/// "week:YYYY-W##", "day:YYYY-MM-DD" — all JST-anchored with 02:00 day-boundary.
+/// Single source of truth for total_count / done_number on every wire shape.
+/// </summary>
+public class ViewerEventCounter
+{
+    public long ViewerId { get; set; }
+    public string EventKey { get; set; } = "";
+    public string Period { get; set; } = "";
+    public int Count { get; set; }
+}