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/store/index.ts

@@ -7,7 +7,6 @@
 
 import { create } from "zustand";
 import type {
-  CategoryUnit,
   Material,
   Project,
   Quote,
@@ -15,7 +14,6 @@ import type {
   Store,
   Supplier,
   Topic,
-  UnitConversion,
 } from "../lib/types";
 
 interface AppStore {
@@ -24,8 +22,6 @@ interface AppStore {
   materials: Material[];
   projects: Project[];
   quotes: Quote[];
-  categoryUnits: CategoryUnit[];
-  unitConversions: UnitConversion[];
   standardConversions: StandardConversion[];
   topics: Topic[]; // دسته‌بندی‌های ۸گانه — از /material-categories
 
@@ -45,9 +41,6 @@ interface AppStore {
   deleteSupplier: (id: string) => void;
   addMaterial: (m: Material) => void;
   addProject: (p: Project) => void;
-  setCategoryUnit: (cu: CategoryUnit) => void;
-  setConversion: (c: UnitConversion) => void;
-  deleteConversion: (id: string) => void;
   setStandardConversion: (sc: StandardConversion) => void;
   deleteStandardConversion: (id: string) => void;
 
@@ -62,8 +55,6 @@ export const useAppStore = create<AppStore>()((set) => ({
   materials: [],
   projects: [],
   quotes: [],
-  categoryUnits: [],
-  unitConversions: [],
   standardConversions: [],
   topics: [],
 
@@ -77,8 +68,6 @@ export const useAppStore = create<AppStore>()((set) => ({
       materials: store.materials,
       projects: store.projects,
       quotes: store.quotes,
-      categoryUnits: store.categoryUnits,
-      unitConversions: store.unitConversions,
       standardConversions: store.standardConversions,
       topics: store.topics,
     }),
@@ -96,24 +85,6 @@ export const useAppStore = create<AppStore>()((set) => ({
   deleteSupplier: (id) => set((s) => ({ suppliers: s.suppliers.filter((x) => x.id !== id) })),
   addMaterial: (m) => set((s) => ({ materials: [...s.materials, m] })),
   addProject: (p) => set((s) => ({ projects: [...s.projects, p] })),
-  setCategoryUnit: (cu) =>
-    set((s) => {
-      const idx = s.categoryUnits.findIndex((x) => x.id === cu.id);
-      const categoryUnits = [...s.categoryUnits];
-      if (idx >= 0) categoryUnits[idx] = cu;
-      else categoryUnits.push(cu);
-      return { categoryUnits };
-    }),
-  setConversion: (c) =>
-    set((s) => {
-      const idx = s.unitConversions.findIndex((x) => x.id === c.id);
-      const unitConversions = [...s.unitConversions];
-      if (idx >= 0) unitConversions[idx] = c;
-      else unitConversions.push(c);
-      return { unitConversions };
-    }),
-  deleteConversion: (id) =>
-    set((s) => ({ unitConversions: s.unitConversions.filter((x) => x.id !== id) })),
   setStandardConversion: (sc) =>
     set((s) => {
       const idx = s.standardConversions.findIndex((x) => x.id === sc.id);