Inventory: Rows Only Exist When There's Actual Stock

Creating a product no longer creates an empty inventory row at every warehouse location. Missing rows are treated as zero everywhere, and a row is created on demand the first time real stock or an allocation lands at a location. This removes a long-standing tax on product creation and brand setup, and clears the way for faster Inventory, Products, and Buildings pages by getting rid of millions of empty rows that were never operationally meaningful.

The filtered Inventory page — when narrowed to a specific building or location — now lists only items that actually have a row at that location. The unfiltered Inventory page is unchanged: it still lists every item, with zero where nothing is on hand.