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/materials.py

@@ -18,7 +18,7 @@ from app.models.catalog import (
     MaterialUnit,
     Unit,
 )
-from app.models.procurement import LineItem, Supplier, SupplierMaterial
+from app.models.procurement import LineItem
 from app.schemas.catalog import (
     MaterialAttributeValuesPut,
     MaterialCreate,
@@ -97,7 +97,6 @@ async def _apply_attr_filters(db: AsyncSession, stmt, request: Request):
 async def list_materials(
     page: int = Query(1, ge=1),
     page_size: int = Query(20, ge=1, le=100),
-    category_id=None,
     brand_id=None,
     status: str | None = None,
     q: str | None = None,
@@ -107,17 +106,6 @@ async def list_materials(
     db: AsyncSession = Depends(get_db),
 ):
     stmt = select(Material)
-    if category_id:
-        sub = (
-            select(1)
-            .select_from(SupplierMaterial)
-            .join(Supplier, Supplier.id == SupplierMaterial.supplier_id)
-            .where(
-                SupplierMaterial.material_id == Material.id,
-                Supplier.category_id == category_id,
-            )
-        )
-        stmt = stmt.where(exists(sub))
     if brand_id:
         # WHERE EXISTS brand_materials with that brand_id
         sub = select(1).where(