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>

models/database/vendor.py

@@ -50,10 +50,6 @@ class Vendor(Base, TimestampMixin):
     name = Column(String, nullable=False)  # Non-nullable name column for the vendor (brand name)
     description = Column(Text)  # Optional text description column for the vendor
 
-    owner_user_id = Column(
-        Integer, ForeignKey("users.id"), nullable=True
-    )  # Foreign key to user ID of the vendor's owner (DEPRECATED - use company.owner_user_id instead)
-
     # Letzshop URLs - multi-language support (brand-specific marketplace feeds)
     letzshop_csv_url_fr = Column(String)  # URL for French CSV in Letzshop
     letzshop_csv_url_en = Column(String)  # URL for English CSV in Letzshop
@@ -73,9 +69,6 @@ class Vendor(Base, TimestampMixin):
     company = relationship(
         "Company", back_populates="vendors"
     )  # Relationship with Company model for the parent company
-    owner = relationship(
-        "User", back_populates="owned_vendors"
-    )  # Relationship with User model for the vendor's owner (legacy)
     vendor_users = relationship(
         "VendorUser", back_populates="vendor"
     )  # Relationship with VendorUser model for users in this vendor