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_concurrency.go

@@ -64,8 +64,9 @@ func (s *OpsService) getAccountsLoadMapBestEffort(ctx context.Context, accounts
 		if acc.ID <= 0 {
 			continue
 		}
-		if prev, ok := unique[acc.ID]; !ok || acc.Concurrency > prev {
-			unique[acc.ID] = acc.Concurrency
+		lf := acc.EffectiveLoadFactor()
+		if prev, ok := unique[acc.ID]; !ok || lf > prev {
+			unique[acc.ID] = lf
 		}
 	}