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/services/quote_service.py

@@ -873,7 +873,6 @@ async def quote_stats(
     project_id: UUID | None = None,
     quote_date_from=None,
     quote_date_to=None,
-    category_id: UUID | None = None,
 ) -> dict:
     conditions = []
     if supplier_id is not None:
@@ -884,8 +883,6 @@ async def quote_stats(
         conditions.append(Quote.quote_date >= quote_date_from)
     if quote_date_to is not None:
         conditions.append(Quote.quote_date <= quote_date_to)
-    if category_id is not None:
-        conditions.append(_quote_has_category(category_id))
 
     rows = (
         await db.execute(
@@ -917,18 +914,6 @@ async def quote_stats(
     }
 
 
-def _quote_has_category(category_id: UUID):
-    """EXISTS: quote's supplier belongs to category_id."""
-    sub = (
-        select(1)
-        .where(
-            Supplier.id == Quote.supplier_id,
-            Supplier.category_id == category_id,
-        )
-    )
-    return func.exists(sub)
-
-
 async def project_stats(db: AsyncSession, project_id: UUID) -> dict:
     """Counts + total value per quote status for ONE project, isolated per project.