fix(catalog): resolve material category through the linked supplier (v2.7 follow-up)

v2.7 dropped materials.category_id and the API started returning category: null for
every material, but the frontend still read material.category as the material's label.
Effect: the edit page's material field rendered blank for every row, the dashboard
category filter listed nothing (and matched nothing), the wizard's category suggestions
were empty, and the conversion lookups keyed by category (refConverted →
categoryUnits/unitConversions) silently returned null, dropping the per-reference-unit
price. Saving an edited quote matched materials on an always-true category term, so two
materials sharing brand+specs were interchangeable.

Auditing the dropped column also turned up four places still dereferencing it, one
request away from a 500 and hidden only because the inventory is empty:
GET /inventory (serializer), GET /inventory?category_id=…, GET /inventory/materials/options
and DELETE /material-categories/{id}.

Backend:
- expand_service: category_by_material() resolves a material's display category from its
  FIRST linked supplier (oldest supplier_materials row — the v2.5 trigger's rule), used by
  the material and line-item payloads; category_id is populated again when not expanded.
- inventory: list filter, row serializer and material-options filter match through
  supplier_materials ⋈ suppliers, mirroring GET /materials?category_id=…
- material_categories: the delete guard blocks on suppliers (the RESTRICT FK) and says so.

Frontend:
- Material.category → Material.supplier_category (supplier-derived, documented); the
  rename pushed every read site through tsc.
- Labels use the material's own name; the edit modal field is «متریال» and prefills by
  name; formatMaterialTitle no longer falls back to a category.
- Identity matching (edit modal, wizard, material memory) is name + brand + specs via a
  shared matchMaterial(), so same-brand same-spec materials are not conflated; legacy
  composed names are still found through materialBaseName().
- Filters read the supplier's category (dashboard options + quote filter, consistent with
  the backend's quote filtering); supplier categories now come from the supplier FK
  instead of the removed JSONB column. Conversion keys use supplier_category.
- Removed dead code carrying the old logic: two uncalled topicCategories copies and
  materialOptions' unused "cat" branch.

Verified: both images build clean (tsc -b && vite build); the API was exercised in-process
against a restore of the current production dump with an inventory row added — material
and line-item payloads carry the supplier category, the inventory endpoints that used to
raise return 200, and the category delete guard returns 409 instead of 500.

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 select
+from sqlalchemy import exists, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.auth import UserContext, get_optional_user, require_role
@@ -27,12 +27,13 @@ from app.models.inventory import (
     InventoryTransaction,
     Warehouse,
 )
-from app.models.procurement import Project
+from app.models.procurement import Project, Supplier, SupplierMaterial
 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,
@@ -333,8 +334,18 @@ async def list_inventory(
             Warehouse.project_id == project_id
         )
     if category_id is not None:
-        stmt = stmt.join(Material, Material.id == InventoryItem.material_id).where(
-            Material.category_id == category_id
+        # 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)
@@ -357,10 +368,8 @@ 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()}
-    categories = {}
-    if materials:
-        cat_ids = {m.category_id for m in materials.values()}
-        categories = {c.id: c for c in (await db.execute(select(MaterialCategory).where(MaterialCategory.id.in_(cat_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()}
@@ -370,7 +379,7 @@ async def _serialize_inventory_rows(db: AsyncSession, items: list[InventoryItem]
     return [
         inventory_to_dict(
             i, warehouses[i.warehouse_id], materials[i.material_id],
-            categories.get(materials[i.material_id].category_id), brands.get(i.brand_id), units.get(i.unit_id),
+            material_category.get(i.material_id), brands.get(i.brand_id), units.get(i.unit_id),
         )
         for i in items
     ]
@@ -556,7 +565,20 @@ async def material_options(
     if category is None:
         raise NotFoundError("دسته‌بندی یافت نشد", field="category_id")
 
-    stmt = select(Material).where(Material.category_id == category_id, Material.status == "active")
+    # 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: