refactor(catalog): reduce the category taxonomy to a supplier label

Categories are kept only as the label each supplier carries (suppliers.category_id
+ the material_categories list it points at). Everything that treated category as
a shared key across materials, inventory, quotes and unit conversions is removed —
including two features that were never populated (0 rows on every volume).

Removed (backend):
- routers category_attributes, category_ref_units, unit_conversions + registration
- ?category_id= filters on /materials, /inventory and /quotes (the /suppliers one
  stays — it filters suppliers by their own label)
- GET /inventory/materials/options: required a category and had no caller
- the material-payload category derivation (expand_service.category_by_material,
  the inventory serializer lookup) and the material category on /pm/on-the-way;
  the wire keys stay, now always null, so the payload shape is unchanged
- startup DDL: DROP the empty category_attributes / category_reference_units /
  unit_conversions tables (idempotent) and their CREATE statements; seed.py
  truncate list updated

Removed (frontend):
- the category-keyed conversion path: refUnitForCategory / findConversion /
  refConverted, the categoryUnits + unitConversions store slices, api clients,
  hooks and the "≈ converted quantity" line on quote detail and project pages
  (it needed rows in those tables to render, and there were none)
- material.supplier_category and its read sites (the edit modal grew one
  yesterday to restore labels; the label lives on the supplier now)
- the inventory page's category chips, the row filter and the «دسته» column,
  plus the lookup's categories fetch and catName helper

Kept: supplier label + its list and picker, /suppliers?category_id=, the global
standard_conversions feature and the units page (material_units conversions).

Verified against a restore of the production dump: startup DDL drops exactly the
three tables, suppliers keep their label (3/3), material_categories still serves
its 8 rows, the removed endpoints 404, and materials/inventory/quotes/units/
standard-conversions all return 200 with quote line items intact.

frontend/src/pages/Suppliers/SupplierDetailPage.tsx

@@ -48,21 +48,8 @@ export function SupplierDetailPage() {
     return [...set].sort((a, b) => a.localeCompare(b, "fa"));
   }, [materials]);
 
-  // متریال‌های خریداری‌شده از این تأمین‌کننده (از پیشنهادها)
-  const purchased = useMemo(() => {
-    const out = new Set<string>();
-    quotes.forEach((q) => {
-      if (q.supplier_id !== id) return;
-      (q.line_items || []).forEach((li) => {
-        if (li.rejected) return;
-        const m = materials.find((x) => x.id === li.material_id);
-        // دسته‌ی نمایشی متریال = دسته‌ی نخستین تأمین‌کننده‌ی متصل (v2.7)
-        if (!m || !m.supplier_category) return;
-        out.add(m.supplier_category);
-      });
-    });
-    return [...out].sort((a, b) => a.localeCompare(b, "fa"));
-  }, [quotes, materials, id]);
+  // دسته‌ی این تأمین‌کننده — دسته فقط برچسبِ خودِ تأمین‌کننده است (v2.7)
+  const purchased = useMemo(() => [...(s?.categories || [])].sort((a, b) => a.localeCompare(b, "fa")), [s]);
 
   if (!s) {
     return (