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/Projects/ProjectsPage.tsx

@@ -19,7 +19,6 @@ import {
   lineItemTotal,
   materialById,
   materialLabel,
-  refConverted,
   supplierById,
 } from "../../lib/quoteUtils";
 import { faNum, fmtDate } from "../../lib/formatters";
@@ -81,11 +80,11 @@ export function ProjectsPage() {
   const quotes = useQuotes();
   const suppliers = useSuppliers();
   const materials = useMaterials();
-  const { categoryUnits, unitConversions, standardConversions } = useUnits();
+  const { standardConversions } = useUnits();
   const addProject = useAddProject();
   const topics = useAppStore((s) => s.topics);
 
-  const store: Store = { quotes, suppliers, materials, projects, categoryUnits, unitConversions, standardConversions, topics };
+  const store: Store = { quotes, suppliers, materials, projects, standardConversions, topics };
 
   const [openProject, setOpenProject] = useState<string | null>(null);
   const [newName, setNewName] = useState("");
@@ -189,17 +188,10 @@ function renderProjectDetail(store: Store, name: string): React.ReactElement {
 
   const itemLine = (li: Quote["line_items"][number]) => {
     const total = lineItemTotal(li);
-    const conv = refConverted(store, li);
     return (
       <div key={li.id}>
         {matName(li.material_id)} — {faNum(li.quantity)} {li.unit} × {faNum(li.unit_price || 0)} ={" "}
         <b>{faNum(total || 0)} تومان</b>
-        {conv && (
-          <span className="pj-conv">
-            {" "}
-            — <span className="pj-conv">≈ {faNum(conv.qty)} {conv.unit}({faNum(conv.perRef ?? 0)} تومان/{conv.unit})</span>
-          </span>
-        )}
       </div>
     );
   };