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/channelserver/repo_mocks_test.go

@@ -810,8 +810,6 @@ type mockGachaRepo struct {
 	allEntriesErr error
 	weightDivisor float64
 
-	// FrontierPoints from gacha
-	addFPErr error
 }
 
 func (m *mockGachaRepo) GetEntryForTransaction(_ uint32, _ uint8) (uint8, uint16, int, error) {