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/ViewerPlayedTogether.cs

@@ -0,0 +1,17 @@
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One row per (owner, opponent) pair. Upserted on each new battle so the table
+/// holds at most one row per opponent. Per-viewer 50-row retention cap pruned
+/// by <c>IPlayedTogetherWriter.RecordAsync</c>.
+/// </summary>
+public class ViewerPlayedTogether
+{
+    public long OwnerViewerId { get; set; }
+    public long OpponentViewerId { get; set; }
+    public DateTime PlayedAt { get; set; }
+    public int PlayedMode { get; set; }
+    public int BattleType { get; set; }
+    public int DeckFormat { get; set; }
+    public int TwoPickType { get; set; }
+}