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/IFriendService.cs

@@ -0,0 +1,30 @@
+namespace SVSim.Database.Services.Friend;
+
+public interface IFriendService
+{
+    Task<FriendInfoResult> GetFriendsAsync(long viewerId, CancellationToken ct);
+    Task<ReceiveApplyInfoResult> GetReceiveAppliesAsync(long viewerId, CancellationToken ct);
+    Task<SendApplyInfoResult> GetSendAppliesAsync(long viewerId, CancellationToken ct);
+    Task<PlayedTogetherResult> GetPlayedTogetherAsync(long viewerId, CancellationToken ct);
+
+    /// <summary>Returns null when not found, self-search, or any error.</summary>
+    Task<FriendEntry?> SearchAsync(long viewerId, int targetViewerId, CancellationToken ct);
+
+    /// <summary>No-op if target missing, self, already friends, already-pending apply, or at outgoing-apply cap.</summary>
+    Task SendApplyAsync(long viewerId, int targetViewerId, CancellationToken ct);
+
+    /// <summary>No-op if apply not addressed to caller, would push either side past friend cap. Cleans reverse-direction apply if present.</summary>
+    Task ApproveApplyAsync(long viewerId, int applyId, CancellationToken ct);
+
+    /// <summary>No-op if apply not addressed to caller.</summary>
+    Task RejectApplyAsync(long viewerId, int applyId, CancellationToken ct);
+
+    /// <summary>No-op if apply not sent by caller.</summary>
+    Task CancelApplyAsync(long viewerId, int applyId, CancellationToken ct);
+
+    Task RejectAllAppliesAsync(long viewerId, CancellationToken ct);
+    Task CancelAllAppliesAsync(long viewerId, CancellationToken ct);
+
+    /// <summary>Deletes both directions of the friendship (A→B and B→A).</summary>
+    Task RejectFriendAsync(long viewerId, int targetViewerId, CancellationToken ct);
+}