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

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

SVSim.Database/Models/ViewerAchievement.cs

@@ -0,0 +1,17 @@
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// Per-viewer state for one achievement type. Composite PK (ViewerId, AchievementType) configured
+/// in DbContext. <c>Level</c> is the viewer's current tier; <c>max_level</c> on the wire is
+/// derived from catalog as MAX(Level) per type. Lazy-created at /load/index time — one row per
+/// AchievementCatalogEntries.AchievementType that the viewer doesn't yet have a row for.
+/// </summary>
+public class ViewerAchievement
+{
+    public long ViewerId { get; set; }
+    public int AchievementType { get; set; }
+    public int Level { get; set; } = 1;
+    public int AchievementStatus { get; set; }
+    public int NowAchievedLevel { get; set; }
+    public int ResultAnnounceSawLevel { get; set; }
+}