feat(items): add base price and support usage in creation defaults

Add `base_price` to items schema, API, and edit history
Display and edit base price in item details and forms
Add `clearable` prop to TagsSelect for optional single selections
Include usage in item creation session defaults

frontend/src/services/api.ts

@@ -257,6 +257,7 @@ export interface Item extends EditInfo {
   baseName?: string;
   details: string;
   baseDetails?: string;
+  basePrice: number | null;
   isEventItem: boolean;
   translations?: TranslationMap;
   image: EntityImage | null;
@@ -789,6 +790,7 @@ export interface PokemonImageOptionsResult {
 export interface ItemPayload {
   name: string;
   details: string;
+  basePrice: number | null;
   translations?: TranslationMap;
   categoryId: number;
   usageId: number | null;