fix: add prefix field to TenantUpdate schema

prefix was missing from TenantUpdate, causing updates via Admin Portal
to silently ignore the prefix value.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/tenant.py

@@ -33,6 +33,7 @@ class TenantCreate(TenantBase):
 
 
 class TenantUpdate(BaseModel):
+    prefix: Optional[str] = None
     name: Optional[str] = None
     name_eng: Optional[str] = None
     tax_id: Optional[str] = None