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/Inventory/useInventoryData.ts

@@ -9,10 +9,9 @@ import { useDataChanged } from "../../lib/crossTab";
 import type { InventoryRow, InventoryTransaction, Warehouse } from "../../lib/types";
 
 export interface LookupData {
-  categories: { id: string; slug: string; name: string }[];
   units: { id: number; code: string; name_fa: string | null }[];
   brands: { id: string; name: string }[];
-  materials: { id: string; name: string; category: { id: string; name: string } | null; canonical_unit: { id: number; code: string; name_fa: string | null } | null }[];
+  materials: { id: string; name: string; canonical_unit: { id: number; code: string; name_fa: string | null } | null }[];
   projects: { id: string; name: string; pm_user_id: string | null; pm_name?: string | null }[];
 }
 
@@ -34,14 +33,13 @@ export function useInventoryData(): InventoryData {
   const [inventory, setInventory] = useState<InventoryRow[]>([]);
   const [transactions, setTransactions] = useState<InventoryTransaction[]>([]);
   const [lookup, setLookup] = useState<LookupData>({
-    categories: [], units: [], brands: [], materials: [], projects: [],
+    units: [], brands: [], materials: [], projects: [],
   });
   const [loading, setLoading] = useState(true);
 
   const load = useCallback(async () => {
     try {
-      const [cats, units, brands, mats, projs, whs, inv, txs] = await Promise.all([
-        apiV26.lookup.categories(),
+      const [units, brands, mats, projs, whs, inv, txs] = await Promise.all([
         apiV26.lookup.units(),
         apiV26.lookup.brands(),
         apiV26.lookup.materials(),
@@ -50,7 +48,7 @@ export function useInventoryData(): InventoryData {
         apiV26.inventory.list({ project_id: projectId }),
         apiV26.inventory.transactions({ project_id: projectId }),
       ]);
-      setLookup({ categories: cats, units, brands, materials: mats, projects: projs });
+      setLookup({ units, brands, materials: mats, projects: projs });
       setWarehouses(whs);
       setInventory(inv);
       setTransactions(txs);
@@ -108,11 +106,6 @@ export function materialName(materials: LookupData["materials"], id: string): st
   return m ? m.name : "—";
 }
 
-export function catName(categories: LookupData["categories"], id: string | null | undefined): string {
-  const c = categories.find((x) => x.id === id);
-  return c ? c.name : "—";
-}
-
 export function projName(projects: LookupData["projects"], id: string | null | undefined): string {
   const p = projects.find((x) => x.id === id);
   return p ? p.name : "—";