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/ViewerFriend.cs

@@ -0,0 +1,13 @@
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One row per direction of a friendship. Approving an apply creates two rows
+/// (A → B and B → A). <see cref="FriendViewerId"/> from a played-together row
+/// can be self-joined against this table to detect an existing friendship.
+/// </summary>
+public class ViewerFriend
+{
+    public long OwnerViewerId { get; set; }
+    public long FriendViewerId { get; set; }
+    public DateTime CreatedAt { get; set; }
+}