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/inventory.py

@@ -15,7 +15,7 @@ from dataclasses import dataclass
 from decimal import Decimal
 
 from fastapi import APIRouter, Depends, Query, Request
-from sqlalchemy import exists, select
+from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.auth import UserContext, get_optional_user, require_role
@@ -27,13 +27,12 @@ from app.models.inventory import (
     InventoryTransaction,
     Warehouse,
 )
-from app.models.procurement import Project, Supplier, SupplierMaterial
+from app.models.procurement import Project
 from app.schemas.inventory import (
     InventoryTransactionCreate,
     WarehouseCreate,
     WarehouseUpdate,
 )
-from app.services.expand_service import category_by_material
 from app.services.inventory_service import (
     apply_transaction,
     get_warehouse_or_404,
@@ -315,7 +314,6 @@ async def delete_warehouse(
 @router.get("")
 async def list_inventory(
     project_id: uuid.UUID | None = None,
-    category_id=None,
     warehouse_id=None,
     material_id=None,
     sort: str | None = None,
@@ -333,20 +331,6 @@ async def list_inventory(
         stmt = stmt.join(Warehouse, Warehouse.id == InventoryItem.warehouse_id).where(
             Warehouse.project_id == project_id
         )
-    if category_id is not None:
-        # v2.7: materials carry no category — match through their suppliers
-        # (any link, mirroring GET /materials?category_id=…).
-        stmt = stmt.where(
-            exists(
-                select(1)
-                .select_from(SupplierMaterial)
-                .join(Supplier, Supplier.id == SupplierMaterial.supplier_id)
-                .where(
-                    SupplierMaterial.material_id == InventoryItem.material_id,
-                    Supplier.category_id == category_id,
-                )
-            )
-        )
     if warehouse_id is not None:
         stmt = stmt.where(InventoryItem.warehouse_id == warehouse_id)
     if material_id is not None:
@@ -368,8 +352,6 @@ async def _serialize_inventory_rows(db: AsyncSession, items: list[InventoryItem]
     unit_ids = {i.unit_id for i in items}
     warehouses = {w.id: w for w in (await db.execute(select(Warehouse).where(Warehouse.id.in_(wh_ids)))).scalars()}
     materials = {m.id: m for m in (await db.execute(select(Material).where(Material.id.in_(mat_ids)))).scalars()}
-    # v2.7: the category shown for a material comes from its first linked supplier.
-    material_category = await category_by_material(db, mat_ids)
     brands = {}
     if brand_ids:
         brands = {b.id: b for b in (await db.execute(select(Brand).where(Brand.id.in_(brand_ids)))).scalars()}
@@ -379,7 +361,7 @@ async def _serialize_inventory_rows(db: AsyncSession, items: list[InventoryItem]
     return [
         inventory_to_dict(
             i, warehouses[i.warehouse_id], materials[i.material_id],
-            material_category.get(i.material_id), brands.get(i.brand_id), units.get(i.unit_id),
+            None, brands.get(i.brand_id), units.get(i.unit_id),
         )
         for i in items
     ]
@@ -541,71 +523,3 @@ async def transaction_types():
         "PURCHASE", "WITHDRAWAL", "RETURN", "TRANSFER_IN", "TRANSFER_OUT",
         "DAMAGE", "WASTE", "LOSS", "FOUND", "STOCK_COUNT",
     )]}
-
-
-# ---------------------------------------------------------------------------
-# Material selector — filtered by Category + Warehouse (transaction forms)
-# ---------------------------------------------------------------------------
-
-@router.get("/materials/options")
-async def material_options(
-    category_id: uuid.UUID,
-    warehouse_id: uuid.UUID | None = None,
-    scope: Scope = Depends(inventory_scope),
-    db: AsyncSession = Depends(get_db),
-):
-    """Materials for the transaction form.
-
-    Always filtered by category. When `warehouse_id` is given, only materials
-    that already exist in that warehouse's inventory are returned (so the PM
-    works with the actual stock of that warehouse); otherwise the full
-    category list is returned (e.g. first purchase of a new material).
-    """
-    category = await db.get(MaterialCategory, category_id)
-    if category is None:
-        raise NotFoundError("دسته‌بندی یافت نشد", field="category_id")
-
-    # v2.7: a material has no category column — it belongs to a category when a
-    # linked supplier does (same rule as GET /materials?category_id=…).
-    stmt = select(Material).where(
-        Material.status == "active",
-        exists(
-            select(1)
-            .select_from(SupplierMaterial)
-            .join(Supplier, Supplier.id == SupplierMaterial.supplier_id)
-            .where(
-                SupplierMaterial.material_id == Material.id,
-                Supplier.category_id == category_id,
-            )
-        ),
-    )
-    if warehouse_id is not None:
-        wh = await get_warehouse_or_404(db, warehouse_id)
-        if scope.role == ROLE_PM and wh.project_id != scope.project_id:
-            raise ForbiddenError("این انبار متعلق به پروژه شما نیست")
-        stmt = stmt.where(
-            Material.id.in_(
-                select(InventoryItem.material_id).where(InventoryItem.warehouse_id == warehouse_id)
-            )
-        )
-    stmt = stmt.order_by(Material.name)
-    materials = (await db.execute(stmt)).scalars().all()
-
-    # Brands per material (from brand_materials catalog + brands actually used in stock)
-    brand_rows = (await db.execute(
-        select(BrandMaterial.material_id, Brand.id, Brand.name)
-        .join(Brand, Brand.id == BrandMaterial.brand_id)
-        .where(BrandMaterial.material_id.in_([m.id for m in materials]), BrandMaterial.is_active.is_(True))
-    )).all()
-    brands_by_material: dict[str, list[dict]] = {}
-    for mat_id, b_id, b_name in brand_rows:
-        brands_by_material.setdefault(str(mat_id), []).append({"id": str(b_id), "name": b_name})
-
-    return {"data": [
-        {
-            "id": str(m.id),
-            "name": m.name,
-            "brands": brands_by_material.get(str(m.id), []),
-        }
-        for m in materials
-    ]}