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

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

SVSim.Database/Models/ViewerMission.cs

@@ -0,0 +1,24 @@
+using System.ComponentModel.DataAnnotations.Schema;
+using SVSim.Database.Common;
+
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One assigned mission slot for a viewer. <c>Id</c> is the wire <c>UserMission.id</c> — echoed
+/// back as the retire-request parameter, auto-generated. Slot 0 = daily (lot_type=6),
+/// Slots 1..3 = weekly (lot_type=2). Progress (<c>total_count</c> on the wire) is NOT stored
+/// here — it's read from <see cref="ViewerEventCounter"/> at response-build time, keyed by the
+/// catalog row's EventType.
+/// </summary>
+public class ViewerMission : BaseEntity<long>
+{
+    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+    public override long Id { get; set; }
+
+    public long ViewerId { get; set; }
+    public int MissionCatalogId { get; set; }
+    public int Slot { get; set; }
+    public long AssignedAt { get; set; }
+    public long? ClaimedAt { get; set; }
+    public int MissionStatus { get; set; } = 1;
+}