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/Wizard/Step2LineItems.tsx

@@ -77,7 +77,7 @@ export function materialOptions(
   // هم‌نام پیشنهاد می‌شوند.
   if (cat)
     list = list.filter(
-      (m) => normalize(materialBaseName(m.name, m.brand, m.specs, m.supplier_category)) === normalize(cat),
+      (m) => normalize(materialBaseName(m.name, m.brand, m.specs)) === normalize(cat),
     );
   // مشخصه‌ها سراسری هستند: رجیستری attribute-definitions + مشخصه‌های همه‌ی
   // متریال‌ها + مشخصه‌های تایپ‌شده در سایر ردیف‌ها. به تاپیک/دسته/برندِ قلمِ
@@ -109,7 +109,7 @@ export function Step2LineItems({ items, onItemsChange, selectedTopic, onRemember
   const allMaterialNames = useMemo(() => {
     const set = new Set<string>();
     materials.forEach((m) => {
-      const base = materialBaseName(m.name, m.brand, m.specs, m.supplier_category);
+      const base = materialBaseName(m.name, m.brand, m.specs);
       if (base) set.add(base);
     });
     return [...set].sort((a, b) => a.localeCompare(b, "fa"));

frontend/src/pages/Wizard/WizardPage.tsx

@@ -317,7 +317,6 @@ export function WizardPage() {
             brand,
             specs,
             base_unit: unit || "عدد",
-            supplier_category: "",
           });
           materialId = created.id;
         } catch (err) {