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/user.py

@@ -51,7 +51,6 @@ class User(Base, TimestampMixin):
         "MarketplaceImportJob", back_populates="user"
     )
     owned_companies = relationship("Company", back_populates="owner")
-    owned_vendors = relationship("Vendor", back_populates="owner")
     vendor_memberships = relationship(
         "VendorUser", foreign_keys="[VendorUser.user_id]", back_populates="user"
     )
@@ -78,12 +77,20 @@ class User(Base, TimestampMixin):
         return self.role == UserRole.VENDOR.value
 
     def is_owner_of(self, vendor_id: int) -> bool:
-        """Check if user is the owner of a specific vendor."""
-        return any(v.id == vendor_id for v in self.owned_vendors)
+        """
+        Check if user is the owner of a specific vendor.
+
+        Ownership is determined via company ownership:
+        User owns Company → Company has Vendor → User owns Vendor
+        """
+        for company in self.owned_companies:
+            if any(v.id == vendor_id for v in company.vendors):
+                return True
+        return False
 
     def is_member_of(self, vendor_id: int) -> bool:
         """Check if user is a member of a specific vendor (owner or team)."""
-        # Check if owner
+        # Check if owner (via company)
         if self.is_owner_of(vendor_id):
             return True
         # Check if team member
@@ -93,7 +100,7 @@ class User(Base, TimestampMixin):
 
     def get_vendor_role(self, vendor_id: int) -> str:
         """Get user's role within a specific vendor."""
-        # Check if owner
+        # Check if owner (via company)
         if self.is_owner_of(vendor_id):
             return "owner"