feat(backend): 库存三态落库（stock/on_sale/sold）——挂售可控 + 卖光留痕

- inventories 加 status 枚举列（默认 stock，AutoMigrate 存量即全部「库存」）+ 索引
- 出库审核扣光置 sold 不再软删（留痕台账）；退单恢复行置回 stock；盘亏仍软删
- 新接口 PUT /inventory/:id/status（仅收 stock/on_sale，已售行拒改，租户校验）
- List 默认排除已售（显式 status=sold 才显示）+ status 多值筛选
- Summary：SKU/货值/数量排除已售，新增 sold_count
- 公开店铺页/API 只列 on_sale——酒单从全量裸奔改为人工精选
- 测试：卖光置售/部分不改/退单回库/接口边界/跨租户/汇总口径 8 用例
- 设计方案 docs/design/inventory-sale-status.html + db-schema/CLAUDE.md 同步

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJ1g8XV1YhhmHRzhwWEW7o

backend/internal/model/stock.go

@@ -134,6 +134,9 @@ type Inventory struct {
 	StockInItemID    *uint64 `json:"stock_in_item_id"`
 	InventoryCheckID *uint64 `json:"inventory_check_id"`
 	Quantity         float64 `gorm:"type:decimal(12,3);not null;default:0" json:"quantity"`
+	// Status 库存行三态（2026-07-07）：stock=库存(默认,不公开) / on_sale=在售(公开酒单展示) /
+	// sold=已售(出库审核扣光自动置入,留痕不软删)。stock⇄on_sale 走抽屉 switch；sold 仅出库流程写入。
+	Status string `gorm:"type:enum('stock','on_sale','sold');not null;default:'stock';index:idx_inv_status" json:"status"`
 	// Snapshot fields: copied from product/warehouse/stock-in at approval time.
 	// They reflect the state at the moment of stock-in and are NOT updated when the
 	// referenced product or warehouse record is later modified.