Make subcategory optional and fix category picker for real tenant data

subcategoryid is no longer required to import a catalogue product —
it's a display/grouping hint elsewhere (the codebase already has an
"Uncategorized" fallback for subcategoryid=0), so requiring it was
pure friction with no correctness payoff.

Fixes the category picker at the root: categoryid 2, which tenant
1135's real products actually use, has no row in productcategories at
all (not a filter bug — the master data is genuinely missing it).
Rather than inventing category master data, adds
GET /products/gettenantcategories, which lists categories a tenant's
own products actually use (falling back to a synthesized label when
the master table has no name), so the import category picker always
offers something real instead of an incomplete global list.

Also relaxes the subcategory lookup's tenant filter to include
unowned/global rows (tenantid NULL or 0), not just exact tenant
matches — categoryid 2's real subcategories carry no tenant at all,
so the strict filter was hiding them even when a tenant wanted one.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

models/product.go

@@ -256,6 +256,15 @@ type Subcategory struct {
 	Image           string `json:"image" gorm:"column:image"`
 }
 
+// TenantCategory is a categoryid actually in use by a tenant's own products,
+// with a best-effort name. Used instead of the global productcategories list
+// for the import category picker, since that master table is missing rows
+// for categoryids that are nonetheless in real use (e.g. categoryid 2).
+type TenantCategory struct {
+	Categoryid   int    `json:"categoryid"`
+	Categoryname string `json:"categoryname"`
+}
+
 // ImportedCatalogueRef identifies a catalogue product a tenant has already
 // imported. Brand is always included, even when a caller filtered by a
 // single brand, because a bare catalogueid is ambiguous across brand tables.