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/FriendService.cs

@@ -0,0 +1,69 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+
+namespace SVSim.Database.Services.Friend;
+
+public sealed class FriendService : IFriendService, IPlayedTogetherWriter
+{
+    internal const int FriendMaxCount = 110;
+    internal const int SendApplyMaxCount = 110;
+    internal const int PlayedTogetherRetention = 50;
+
+    // Cosmetic field defaults matching the prod capture's "no campaign, normal player" state.
+    internal const string DefaultDeviceType = "2";
+    internal const string DefaultMaxFriend = "110";
+    internal const string DefaultIsReceivedTwoPickMission = "1";
+    internal const string DefaultBirth = "0";
+    internal const string DefaultMissionChangeTime = "2017-09-15 02:36:09";
+    internal const string DefaultMissionReceiveType = "0";
+    internal const string DefaultIsOfficial = "0";
+    internal const string DefaultIsOfficialMarkDisplayed = "0";
+
+    private readonly SVSimDbContext _db;
+    private readonly ILogger<FriendService> _log;
+
+    public FriendService(SVSimDbContext db, ILogger<FriendService> log)
+    {
+        _db = db;
+        _log = log;
+    }
+
+    public Task<FriendInfoResult> GetFriendsAsync(long viewerId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task<ReceiveApplyInfoResult> GetReceiveAppliesAsync(long viewerId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task<SendApplyInfoResult> GetSendAppliesAsync(long viewerId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task<PlayedTogetherResult> GetPlayedTogetherAsync(long viewerId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task<FriendEntry?> SearchAsync(long viewerId, int targetViewerId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task SendApplyAsync(long viewerId, int targetViewerId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task ApproveApplyAsync(long viewerId, int applyId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task RejectApplyAsync(long viewerId, int applyId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task CancelApplyAsync(long viewerId, int applyId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task RejectAllAppliesAsync(long viewerId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task CancelAllAppliesAsync(long viewerId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task RejectFriendAsync(long viewerId, int targetViewerId, CancellationToken ct) =>
+        throw new NotImplementedException();
+
+    public Task RecordAsync(long ownerViewerId, long opponentViewerId, BattleParticipationContext ctx, CancellationToken ct) =>
+        throw new NotImplementedException();
+}