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/__init__.py

@@ -4,8 +4,6 @@ from app.routers import (
     auth,
     brand_materials,
     brands,
-    category_attributes,
-    category_ref_units,
     files,
     inventory,
     item_decisions,
@@ -24,7 +22,6 @@ from app.routers import (
     supplier_materials,
     suppliers,
     termsheets,
-    unit_conversions,
     units,
     users,
 )
@@ -35,7 +32,6 @@ ROUTERS = [
     brands.router,
     material_categories.router,
     attribute_definitions.router,
-    category_attributes.router,
     materials.router,
     brand_materials.router,
     suppliers.router,
@@ -51,8 +47,6 @@ ROUTERS = [
     item_receipts.router,
     quote_events.router,
     files.router,
-    category_ref_units.router,
-    unit_conversions.router,
     standard_conversions.router,
     inventory.router,
     pm.router,