refactor: update services for company-centric ownership

- Update admin_service: remove vendor owner methods, fix get_all_vendors query
- Update company_service: add transfer_ownership method
- Update vendor_service: check ownership via company relationship
- Remove backwards compatibility code for Vendor.owner_user_id

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/services/company_service.py

@@ -13,10 +13,10 @@ from typing import List, Optional
 from sqlalchemy import func, select
 from sqlalchemy.orm import Session, joinedload
 
-from app.exceptions import CompanyNotFoundException
+from app.exceptions import CompanyNotFoundException, UserNotFoundException
 from models.database.company import Company
 from models.database.user import User
-from models.schema.company import CompanyCreate, CompanyUpdate
+from models.schema.company import CompanyCreate, CompanyTransferOwnership, CompanyUpdate
 
 logger = logging.getLogger(__name__)
 
@@ -110,7 +110,7 @@ class CompanyService:
             select(Company)
             .where(Company.id == company_id)
             .options(joinedload(Company.vendors))
-        ).scalar_one_or_none()
+        ).unique().scalar_one_or_none()
 
         if not company:
             raise CompanyNotFoundException(company_id)
@@ -257,6 +257,69 @@ class CompanyService:
 
         return company
 
+    def transfer_ownership(
+        self,
+        db: Session,
+        company_id: int,
+        transfer_data: CompanyTransferOwnership,
+    ) -> tuple[Company, User, User]:
+        """
+        Transfer company ownership to another user.
+
+        This is a critical operation that:
+        - Changes the company's owner_user_id
+        - All vendors under the company automatically inherit the new owner
+        - Logs the transfer for audit purposes
+
+        Args:
+            db: Database session
+            company_id: Company ID
+            transfer_data: Transfer ownership data
+
+        Returns:
+            Tuple of (company, old_owner, new_owner)
+
+        Raises:
+            CompanyNotFoundException: If company not found
+            UserNotFoundException: If new owner user not found
+            ValueError: If trying to transfer to current owner
+        """
+        # Get company
+        company = self.get_company_by_id(db, company_id)
+        old_owner_id = company.owner_user_id
+
+        # Get old owner
+        old_owner = db.execute(
+            select(User).where(User.id == old_owner_id)
+        ).scalar_one_or_none()
+        if not old_owner:
+            raise UserNotFoundException(str(old_owner_id))
+
+        # Get new owner
+        new_owner = db.execute(
+            select(User).where(User.id == transfer_data.new_owner_user_id)
+        ).scalar_one_or_none()
+        if not new_owner:
+            raise UserNotFoundException(str(transfer_data.new_owner_user_id))
+
+        # Prevent transferring to same owner
+        if old_owner_id == transfer_data.new_owner_user_id:
+            raise ValueError("Cannot transfer ownership to the current owner")
+
+        # Update company owner (vendors inherit ownership via company relationship)
+        company.owner_user_id = new_owner.id
+
+        db.flush()
+
+        logger.info(
+            f"Company {company.id} ({company.name}) ownership transferred "
+            f"from user {old_owner.id} ({old_owner.email}) "
+            f"to user {new_owner.id} ({new_owner.email}). "
+            f"Reason: {transfer_data.transfer_reason or 'Not specified'}"
+        )
+
+        return company, old_owner, new_owner
+
     def _generate_temp_password(self, length: int = 12) -> str:
         """Generate secure temporary password."""
         alphabet = string.ascii_letters + string.digits + "!@#$%^&*"