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/Dashboard/QuoteDetail.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useState } from "react";
-import { Package, Scale, CreditCard, CalendarDays, Paperclip, Building2, Ban } from "lucide-react";
+import { Package, CreditCard, CalendarDays, Paperclip, Building2, Ban } from "lucide-react";
 import type { Attachment, Quote, Store } from "../../lib/types";
 import { api } from "../../lib/api";
 import {
@@ -7,7 +7,6 @@ import {
   itemTermsheetIds,
   lineItemTotal,
   materialById,
-  refConverted,
 } from "../../lib/quoteUtils";
 import { fmtDate, fmtDateTime, fmtNum } from "../../lib/formatters";
 import { groupCommas } from "../../lib/numutil";
@@ -76,7 +75,6 @@ export function QuoteDetail({ store, q }: QuoteDetailProps) {
     const tsIds = itemTermsheetIds(li).filter((tid) => tsList.some((t) => t.id === tid));
     const total = lineItemTotal(li);
     const projs = itemProjects(q, li);
-    const conv = isRejected ? null : refConverted(store, li);
 
     // رسید جزیی → قلم به دو بخش «رسید شده» و «در راه» شکسته می‌شود (نمایشی):
     // مجموع دو بخش همیشه با تعداد/مبلغ اصلی برابر است (عدم تغییر جمع کل پیشنهاد).
@@ -140,12 +138,6 @@ export function QuoteDetail({ store, q }: QuoteDetailProps) {
             {whNames.length > 0 && <span>به انبار: {whNames.join("، ")}</span>}
           </div>
         )}
-        {conv && (
-          <div className="fb-conv">
-            <Scale className="me-1 inline h-3 w-3" /> ≈ {groupCommas(conv.qty, true)} {conv.unit}
-            {conv.perRef != null ? ` · ${groupCommas(conv.perRef, true)} تومان/${conv.unit}` : ""}
-          </div>
-        )}
       </div>
     );
   };