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/expand_service.py

@@ -35,7 +35,6 @@ from app.models.procurement import (
     QuoteEvent,
     QuoteProjects,
     Supplier,
-    SupplierMaterial,
     Termsheet,
     TermsheetScheduleStep,
 )
@@ -203,47 +202,9 @@ def quote_flat(q: Quote) -> dict:
 # Materials
 # ---------------------------------------------------------------------------
 
-async def category_by_material(db: AsyncSession, material_ids) -> dict:
-    """Resolve each material's display category through its FIRST linked supplier.
-
-    v2.7 made materials category-free: ``materials.category_id`` no longer
-    exists and the category lives on the supplier (``suppliers.category_id``).
-    A material linked to several suppliers therefore has no single category of
-    its own — we keep the v2.5 first-link-wins rule (oldest supplier_materials
-    row) so the display value stays stable and matches the old derived column.
-    Materials with no link, or whose first supplier has no category, resolve to
-    nothing (the serializers emit null).
-    """
-    ids = {mid for mid in material_ids if mid is not None}
-    if not ids:
-        return {}
-    links = (
-        await db.execute(
-            select(SupplierMaterial.material_id, Supplier.category_id)
-            .join(Supplier, Supplier.id == SupplierMaterial.supplier_id)
-            .where(SupplierMaterial.material_id.in_(ids))
-            .order_by(SupplierMaterial.created_at)
-        )
-    ).all()
-    first_cat: dict = {}
-    for material_id, category_id in links:
-        if category_id is not None and material_id not in first_cat:
-            first_cat[material_id] = category_id
-    if not first_cat:
-        return {}
-    cats = (
-        await db.execute(select(MaterialCategory).where(MaterialCategory.id.in_(set(first_cat.values()))))
-    ).scalars().all()
-    by_id = {c.id: c for c in cats}
-    return {mid: by_id[cid] for mid, cid in first_cat.items() if cid in by_id}
-
-
 async def _load_material_maps(db: AsyncSession, materials: list[Material], expand: set[str]) -> dict:
     material_ids = [m.id for m in materials]
     maps: dict = {"categories": {}, "units": {}, "brands": {}, "material_brands": {}, "mavs": {}, "attr_defs": {}}
-    # Loaded regardless of expand: the non-expanded branch still returns
-    # category_id, and a silent null there is the bug this replaced.
-    maps["material_categories"] = await category_by_material(db, material_ids)
 
     unit_ids: set[int] = set()
     if "canonical_unit" in expand:
@@ -291,13 +252,12 @@ def _material_to_dict(m: Material, expand: set[str], maps: dict) -> dict:
         "created_at": to_iso(m.created_at),
         "updated_at": to_iso(m.updated_at),
     }
-    # v2.7: never the material's own column (it no longer has one) — the
-    # category comes from the first linked supplier via _category_by_material.
-    material_category = maps["material_categories"].get(m.id)
+    # Category is a supplier attribute now, not a material one — the key stays
+    # in the payload (null) so the contract shape is unchanged.
     if "category" in expand:
-        d["category"] = category_brief(material_category)
+        d["category"] = None
     else:
-        d["category_id"] = str(material_category.id) if material_category else None
+        d["category_id"] = None
     if "canonical_unit" in expand:
         d["canonical_unit"] = unit_to_dict(maps["units"].get(m.canonical_unit_id))
     else:
@@ -340,8 +300,6 @@ class _LineItemData:
     def __init__(self):
         self.materials: dict[uuid.UUID, Material] = {}
         self.categories: dict[uuid.UUID, MaterialCategory] = {}
-        # material id -> its first linked supplier's category (v2.7)
-        self.material_category: dict[uuid.UUID, MaterialCategory] = {}
         self.units: dict[int, Unit] = {}
         self.brands: dict[uuid.UUID, Brand] = {}
         self.material_brands: dict[uuid.UUID, list[BrandMaterial]] = {}
@@ -373,7 +331,6 @@ async def _load_line_item_data(db: AsyncSession, line_items: list[LineItem], exp
         if brand_ids:
             brands = (await db.execute(select(Brand).where(Brand.id.in_(brand_ids)))).scalars().all()
             data.brands = {b.id: b for b in brands}
-        data.material_category = await category_by_material(db, material_ids)
 
     # Pricing always needs item_termsheets in expanded mode.
     its = (await db.execute(select(ItemTermsheet).where(ItemTermsheet.line_item_id.in_(li_ids)))).scalars().all()
@@ -487,7 +444,7 @@ def _line_item_expanded(li: LineItem, expand: set[str], data: _LineItemData, inc
             d["material"] = {
                 "id": str(m.id),
                 "name": m.name,
-                "category": category_brief(data.material_category.get(m.id)),
+                "category": None,
                 "brands": [
                     {"id": str(bm.brand_id), "name": data.brands[bm.brand_id].name, "catalog_code": bm.catalog_code}
                     for bm in data.material_brands.get(m.id, [])

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.