fix(guild): final-review remediation — replay_detail flat, FK on leader, transaction wraps, _db extraction

C1: replay_detail — flatten stored payload to data level. ChatReplayDetailTask.Parse() calls
new ReplayDetailInfo(data) which unguardedly accesses data[battleId], data[seed], data[vid1],
etc. Wrapping under replay_info key crashes the client. Controller now returns Ok(JsonElement)
directly so stored battle fields are at data root. Wire-shape test added.

C2: Guild.LeaderViewerId long to long?; add HasOne<Viewer> FK with OnDelete=SetNull; migration
AddGuildLeaderViewerIdFk; all consumers null-guarded with ?? 0L.

C3: BreakupAsync — wrap 6 destructive ops in IDbContextTransaction with InMemory fallback.

C4: CommitJoinAsync — wrap member-add + viewer-guildId-set + invite/request cleanup in
IDbContextTransaction with InMemory fallback. Chat event emitted after commit.

I1: GuildController — remove SVSimDbContext field; inject IViewerRepository + IGuildMemberRepository.
Add IGuildMemberRepository.GetViewerIdsInAGuildAsync (batch guild-membership check).
All _db.Viewers / _db.GuildMembers queries replaced with repo calls.

I2: GuildService — extract 3 _db.Viewers queries: GetEquippedEmblemIdAsync (CreateAsync),
LoadGuildProfileBatchAsync (ListOutgoingInvitesAsync + ListPendingJoinRequestsForMyGuildAsync).
Add GuildMemberProfile record with IsOfficialMarkDisplayed. GetEmblemListAsync for EmblemList.

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

SVSim.Database/SVSimDbContext.cs

@@ -520,6 +520,9 @@ public class SVSimDbContext : DbContext
             e.Property(g => g.Name).HasMaxLength(64);
             e.Property(g => g.Description).HasMaxLength(512);
             e.HasIndex(g => g.Name);                              // name search
+            e.HasOne<Viewer>().WithMany()
+                .HasForeignKey(g => g.LeaderViewerId)
+                .OnDelete(DeleteBehavior.SetNull);
         });
 
         modelBuilder.Entity<GuildMember>(e =>