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/ent/schema/account.go

@@ -97,6 +97,8 @@ func (Account) Fields() []ent.Field {
 		field.Int("concurrency").
 			Default(3),
 
+		field.Int("load_factor").Optional().Nillable(),
+
 		// priority: 账户优先级，数值越小优先级越高
 		// 调度器会优先使用高优先级的账户
 		field.Int("priority").