refactor: remove Vendor.owner_user_id column

- Remove owner_user_id column and owner relationship from Vendor model
- Update User model ownership checks to use company relationship
- Add migration to drop owner_user_id column from vendors table

Ownership is now determined solely via vendor.company.owner_user_id

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

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

alembic/versions/9f3a25ea4991_remove_vendor_owner_user_id_column.py

@@ -0,0 +1,60 @@
+"""remove_vendor_owner_user_id_column
+
+Revision ID: 9f3a25ea4991
+Revises: 5818330181a5
+Create Date: 2025-12-02 17:58:45.663338
+
+This migration removes the owner_user_id column from the vendors table.
+
+Architecture Change:
+- OLD: Each vendor had its own owner (vendor.owner_user_id)
+- NEW: Vendors belong to a company, company has one owner (company.owner_user_id)
+
+The vendor ownership is now determined via the company relationship:
+- vendor.company.owner_user_id contains the owner
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '9f3a25ea4991'
+down_revision: Union[str, None] = '5818330181a5'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """
+    Remove owner_user_id column from vendors table.
+
+    Ownership is now determined via the company relationship.
+
+    Note: SQLite batch mode recreates the table without the column,
+    so we don't need to explicitly drop constraints.
+    """
+    with op.batch_alter_table('vendors', schema=None) as batch_op:
+        # Drop the column - batch mode handles constraints automatically
+        batch_op.drop_column('owner_user_id')
+
+
+def downgrade() -> None:
+    """
+    Re-add owner_user_id column to vendors table.
+
+    WARNING: This will add the column back but NOT restore the data.
+    You will need to manually populate owner_user_id from company.owner_user_id
+    if reverting this migration.
+    """
+    with op.batch_alter_table('vendors', schema=None) as batch_op:
+        batch_op.add_column(
+            sa.Column('owner_user_id', sa.Integer(), nullable=True)
+        )
+        batch_op.create_foreign_key(
+            'vendors_owner_user_id_fkey',
+            'users',
+            ['owner_user_id'],
+            ['id']
+        )