Implement secretbox handlers and align gacha behavior

- Add secretbox API schemas, handlers, and routes for all/showDetail/pon/multi
- Load secretbox server data through the shared serverdata loader
- Fix rarity mapping and pool rules for SR/SSR/UR special boxes
- Implement thanks festival pool behavior, including UR guarantee and SSR/UR-only rates
- Correct animation-related fields such as is_hit and lowest_rarity, and add rare UR promotion animation

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/handler/banner/helpers.go

@@ -0,0 +1,26 @@
+package banner
+
+import "reflect"
+
+func extractResultField(data any) any {
+	if data == nil {
+		return nil
+	}
+
+	value := reflect.ValueOf(data)
+	if value.Kind() == reflect.Pointer {
+		if value.IsNil() {
+			return nil
+		}
+		value = value.Elem()
+	}
+	if value.Kind() != reflect.Struct {
+		return data
+	}
+
+	field := value.FieldByName("Result")
+	if !field.IsValid() || !field.CanInterface() {
+		return data
+	}
+	return field.Interface()
+}