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/handler/dto/mappers.go

@@ -184,6 +184,7 @@ func AccountFromServiceShallow(a *service.Account) *Account {
 		Extra:                   a.Extra,
 		ProxyID:                 a.ProxyID,
 		Concurrency:             a.Concurrency,
+		LoadFactor:              a.LoadFactor,
 		Priority:                a.Priority,
 		RateMultiplier:          a.BillingRateMultiplier(),
 		Status:                  a.Status,

backend/internal/handler/dto/types.go

@@ -133,6 +133,7 @@ type Account struct {
 	Extra              map[string]any `json:"extra"`
 	ProxyID            *int64         `json:"proxy_id"`
 	Concurrency        int            `json:"concurrency"`
+	LoadFactor         *int           `json:"load_factor,omitempty"`
 	Priority           int            `json:"priority"`
 	RateMultiplier     float64        `json:"rate_multiplier"`
 	Status             string         `json:"status"`