feat(friend): IFriendService + IPlayedTogetherWriter + DTO records

Task 3: service contract (interface + DTOs) and FriendService skeleton.
All methods throw NotImplementedException; Tasks 4-6 fill in the logic.

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

SVSim.Database/Services/Friend/IPlayedTogetherWriter.cs

@@ -0,0 +1,11 @@
+namespace SVSim.Database.Services.Friend;
+
+/// <summary>
+/// Records a recent-opponent entry on the owner viewer. Upserts the (owner, opponent)
+/// row to PlayedAt = now, enforces a 50-row per-viewer retention cap by deleting the
+/// owner's oldest row when at cap. No-op if owner equals opponent.
+/// </summary>
+public interface IPlayedTogetherWriter
+{
+    Task RecordAsync(long ownerViewerId, long opponentViewerId, BattleParticipationContext ctx, CancellationToken ct);
+}