feat(guild): MyPage guild_notification + load_index audit

- Replace GuildNotification TODO stub in MyPageController.Index with
  BuildGuildNotificationAsync: populates guild_id, guild_room_message_id,
  is_join_request, is_invited from real DB state.
- Inject IGuildService, IGuildInviteRepository, IGuildJoinRequestRepository,
  IGuildChatMessageRepository into MyPageController.
- Add GetMaxMessageIdSafelyAsync to IGuildChatMessageRepository (returns null
  when no messages exist, vs GetMaxMessageIdAsync which returns 0).
- LoadDetail.cs audit: grep returned no guild fields — no LoadController changes.
- Add GuildCrossSurfaceTests (4 tests): guild member, pending invite, pending
  join request, no-guild viewer all verified green.

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

SVSim.Database/Repositories/Guild/GuildChatMessageRepository.cs

@@ -142,6 +142,12 @@ public sealed class GuildChatMessageRepository : IGuildChatMessageRepository
         return max ?? 0;
     }
 
+    public async Task<long?> GetMaxMessageIdSafelyAsync(int guildId, CancellationToken ct = default)
+    {
+        var v = await GetMaxMessageIdAsync(guildId, ct);
+        return v == 0 ? null : (long?)v;
+    }
+
     public async Task DeleteAllForGuildAsync(int guildId, CancellationToken ct = default)
     {
         var messages = await _db.GuildChatMessages.Where(m => m.GuildId == guildId).ToListAsync(ct);