fix: re-enable CI lint job and fix ~65 lint errors (partial)

Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.

server/channelserver/handlers_character_test.go

@@ -572,7 +572,7 @@ func BenchmarkCompress(b *testing.B) {
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
-		save.Compress()
+		_ = save.Compress()
 	}
 }
 
@@ -587,6 +587,6 @@ func BenchmarkDecompress(b *testing.B) {
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
-		save.Decompress()
+		_ = save.Decompress()
 	}
 }