fix(account): prevent quota-exceeded API key/Bedrock accounts from being scheduled

Add quota exceeded check to IsSchedulable() and refactor
shouldClearStickySession to delegate to IsSchedulable(), eliminating
duplicated logic and fixing missed overload/rate-limit/expired checks.
Frontend displays quota exceeded status independently via quota fields.

backend/internal/service/account.go

@@ -121,6 +121,9 @@ func (a *Account) IsSchedulable() bool {
 	if a.TempUnschedulableUntil != nil && now.Before(*a.TempUnschedulableUntil) {
 		return false
 	}
+	if a.IsAPIKeyOrBedrock() && a.IsQuotaExceeded() {
+		return false
+	}
 	return true
 }