fix(materials): remove nested db.begin — material PATCH was 500

The router loads the material row (starting a transaction) before calling
material_service.update_material, which then opened its own
async with db.begin() → SQLAlchemy 'A transaction is already begun on
this Session' → INTERNAL_ERROR on every PATCH (incl. changing the
reference unit from ضرایب تبدیل). Mutations now run inside the request
transaction committed by the get_db dependency.

backend/app/services/material_service.py

@@ -160,8 +160,12 @@ async def update_material(db: AsyncSession, material: Material, payload: Materia
 
     v2.5/2.6: category is DERIVED (from the first linked supplier) — it is
     no longer part of the update payload.
+
+    NOTE: no ``db.begin()`` here — the router already starts the transaction
+    (it loads the row first), and a nested begin raises
+    "A transaction is already begun on this Session" (500). The request
+    session commits via the get_db dependency.
     """
-    async with db.begin():
     if payload.name is not None:
         duplicate = await db.scalar(
             select(Material.id).where(