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.

backend/app/routers/pm.py

@@ -38,7 +38,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.auth import UserContext, get_optional_user
 from app.config import settings
 from app.db.engine import get_db
-from app.models.catalog import BrandMaterial, Material, MaterialCategory, Unit
+from app.models.catalog import BrandMaterial, Material, Unit
 from app.models.inventory import InventoryItem, InventoryTransaction, Warehouse
 from app.models.procurement import (
     ItemDecision,
@@ -201,12 +201,6 @@ async def pm_on_the_way(
     # unit / category lookups for display
     unit_ids = {d.LineItem.unit_id for d in decisions}
     units = {u.id: u for u in (await db.execute(select(Unit).where(Unit.id.in_(unit_ids)))).scalars()}
-    sup_cat_ids = {d.Supplier.category_id for d in decisions if d.Supplier.category_id}
-    cats = {}
-    if sup_cat_ids:
-        rows = (await db.execute(select(MaterialCategory).where(MaterialCategory.id.in_(sup_cat_ids)))).scalars().all()
-        cats = {c.id: c for c in rows}
-
     out = []
     for d, li, q, m, sup, proj in decisions:
         received = received_map.get(d.id)
@@ -219,8 +213,7 @@ async def pm_on_the_way(
             "supplier": {"id": str(sup.id), "name": sup.name},
             "project": {"id": str(proj.id), "name": proj.name} if proj else None,
             "decided_at": to_iso(d.decided_at),
-            "material": {"id": str(m.id), "name": m.name,
-                         "category": {"id": str(c.id), "name": c.name} if (c := cats.get(sup.category_id)) else None},
+            "material": {"id": str(m.id), "name": m.name},
             "unit": {"id": units[li.unit_id].id, "code": units[li.unit_id].code,
                      "name_fa": units[li.unit_id].name_fa} if li.unit_id in units else None,
             "ordered_qty": str(li.quantity),