fix(channelserver): handle silently discarded errors across handlers

Replace ~17 instances of '_ =' / '_ :=' with proper error checks that
log warnings or send fail ACKs. Affected handlers: cafe, distitem, data,
guild, guild_board, guild_cooking, guild_scout, house, mercenary, misc,
and rengoku. Also resolves all pre-existing lint issues: unchecked
bf.Seek in tests, unused filtered slice in svc_festa, unused mock
fields, and unused signserver test helper.

server/signserver/repo_mocks_test.go

@@ -253,11 +253,3 @@ func (m *mockSignSessionRepo) GetPSNIDByToken(token string) (string, error) {
 	return m.psnIDByToken, m.psnIDByTokenErr
 }
 
-// newTestServer creates a Server with mock repos for testing.
-func newTestServer(userRepo SignUserRepo, charRepo SignCharacterRepo, sessionRepo SignSessionRepo) *Server {
-	return &Server{
-		userRepo:    userRepo,
-		charRepo:    charRepo,
-		sessionRepo: sessionRepo,
-	}
-}