test(channelserver): replace time.Sleep with polling loops

Blind sleeps accumulate serially (no t.Parallel anywhere) and inflate
under the race detector's scheduling overhead — contributing to the
~136s channelserver test run time.

Replace ~75 arbitrary sleeps (50ms–1s) across 7 test files with 2s
polling loops that exit as soon as the expected condition holds. Sleeps
that are genuinely intentional (race-condition stress tests, cache
expiry, temporal spacing in timestamp tests, backpressure pacing) are
left untouched.

server/channelserver/handlers_savedata_integration_test.go

@@ -3,7 +3,6 @@ package channelserver
 import (
 	"bytes"
 	"testing"
-	"time"
 
 	"erupe-ce/common/mhfitem"
 	cfg "erupe-ce/config"
@@ -617,9 +616,6 @@ func TestPlateDataPersistenceDuringLogout(t *testing.T) {
 	t.Log("Triggering logout via logoutPlayer")
 	logoutPlayer(session)
 
-	// Give logout time to complete
-	time.Sleep(100 * time.Millisecond)
-
 	// ===== VERIFICATION: Check all plate data was saved =====
 	t.Log("--- Verifying plate data persisted ---")