feat(friend): add ViewerFriend + ViewerFriendApply + ViewerPlayedTogether entities

Lays the persistence foundation for the /friend/* API surface. Three new
model classes with composite PKs / unique constraints / FK cascades registered
on SVSimDbContext; 4/4 persistence tests pass on SQLite in-memory.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.Database/Models/ViewerFriendApply.cs

@@ -0,0 +1,17 @@
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One pending friend application. <see cref="Id"/> is the wire <c>apply_id</c>
+/// (auto-generated). Unique on <c>(FromViewerId, ToViewerId)</c> — a viewer can only
+/// have one outstanding apply to any given target.
+/// </summary>
+public class ViewerFriendApply
+{
+    public int Id { get; set; }
+    public long FromViewerId { get; set; }
+    public long ToViewerId { get; set; }
+    public DateTime CreatedAt { get; set; }
+
+    /// <summary>Beginner-friend campaign tag. Defaults to 0 (no campaign). Surfaces as optional <c>mission_type</c> on the wire.</summary>
+    public int MissionType { get; set; }
+}