feat: add full/empty weight fields to Product and last_weighed_at to ProductInventory

Adds full_weight_g and empty_weight_g to ProductBase (inherited by Product
and response models) so per-product package weight specs are captured.
Adds last_weighed_at to ProductInventory to record when a package was last
weighed. Wires up all fields through API schemas, frontend types, forms, and
the product detail page (add/edit/display).

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

frontend/src/lib/api.ts

@@ -69,6 +69,9 @@ export const createInventory = (
 	productId: string,
 	body: Record<string, unknown>
 ): Promise<ProductInventory> => api.post(`/products/${productId}/inventory`, body);
+export const updateInventory = (id: string, body: Record<string, unknown>): Promise<ProductInventory> =>
+	api.patch(`/inventory/${id}`, body);
+export const deleteInventory = (id: string): Promise<void> => api.del(`/inventory/${id}`);
 
 // ─── Routines ────────────────────────────────────────────────────────────────