feat: add independent load_factor field for scheduling load calculation

- Separate load factor from concurrency: concurrency controls actual
  slot acquisition, load_factor controls load rate calculation
- Add EffectiveLoadFactor() method: LoadFactor > Concurrency > 1
- Add load_factor field to Create/Edit/BulkEdit account forms
- Fix RPM default value: auto-fill 15 when RPM enabled but not set
- Fix stale test compilation errors in server and handler packages

backend/internal/service/ops_metrics_collector.go

@@ -389,13 +389,9 @@ func (c *OpsMetricsCollector) collectConcurrencyQueueDepth(parentCtx context.Con
 		if acc.ID <= 0 {
 			continue
 		}
-		maxConc := acc.Concurrency
-		if maxConc < 0 {
-			maxConc = 0
-		}
 		batch = append(batch, AccountWithConcurrency{
 			ID:             acc.ID,
-			MaxConcurrency: maxConc,
+			MaxConcurrency: acc.EffectiveLoadFactor(),
 		})
 	}
 	if len(batch) == 0 {