fix: 批量改进 (#2/#23/#38/#39/#40/#47)

- Web: 隐藏 ICP 备案号（条件渲染），联系入口改为微信咨询（条件渲染，配置 wechat 字段后生效）
- backend: 拼音回填抽取为独立 cmd/backfill-pinyin 工具，不再在启动时运行
- backend: DB 连接池参数（max_idle_conns/max_open_conns）改为配置可控，默认 10/100
- backend: Inventory 反范式字段添加注释说明快照语义
- backend: 全量 handler 采用 util.RespondSuccess/RespondCreated 统一响应格式（51 处）

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/model/stock.go

@@ -91,16 +91,19 @@ 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"`
-	ProductCode      string     `gorm:"size:50" json:"product_code"`
-	ProductName      string     `gorm:"size:200" json:"product_name"`
-	Series           string     `gorm:"size:100" json:"series"`
-	Spec             string     `gorm:"size:100" json:"spec"`
-	Unit             string     `gorm:"size:20" json:"unit"`
-	WarehouseName    string     `gorm:"size:100" json:"warehouse_name"`
-	UnitPrice        *float64   `gorm:"type:decimal(16,2)" json:"unit_price"`
-	ProductionDate   *Date      `gorm:"type:date" json:"production_date"`
-	BatchNo          string     `gorm:"size:50" json:"batch_no"`
-	SupplierName     string     `gorm:"size:200" json:"supplier_name"`
+	// 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.
+	ProductCode    string   `gorm:"size:50" json:"product_code"`
+	ProductName    string   `gorm:"size:200" json:"product_name"`
+	Series         string   `gorm:"size:100" json:"series"`
+	Spec           string   `gorm:"size:100" json:"spec"`
+	Unit           string   `gorm:"size:20" json:"unit"`
+	WarehouseName  string   `gorm:"size:100" json:"warehouse_name"`
+	UnitPrice      *float64 `gorm:"type:decimal(16,2)" json:"unit_price"`
+	ProductionDate *Date    `gorm:"type:date" json:"production_date"`
+	BatchNo        string   `gorm:"size:50" json:"batch_no"`
+	SupplierName   string   `gorm:"size:200" json:"supplier_name"`
 	Remark           string     `gorm:"size:500" json:"remark"`
 	CreatedAt        time.Time  `json:"created_at"`
 	UpdatedAt        time.Time  `json:"updated_at"`