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/account_service.go

@@ -82,6 +82,7 @@ type AccountBulkUpdate struct {
 	Concurrency    *int
 	Priority       *int
 	RateMultiplier *float64
+	LoadFactor     *int
 	Status         *string
 	Schedulable    *bool
 	Credentials    map[string]any