feat: update API endpoints for company-vendor architecture

- Add transfer-ownership endpoint to companies API
- Add user search endpoint for autocomplete (/admin/users/search)
- Update company detail endpoint to include owner info and vendors list
- Update vendor endpoints to use company relationship for ownership
- Update deps.py vendor access check to use company.owner_user_id

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

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

app/api/v1/vendor/auth.py

@@ -91,8 +91,8 @@ def vendor_login(
     vendor_role = "Member"
 
     if vendor:
-        # Check if user is vendor owner
-        is_owner = vendor.owner_user_id == user.id
+        # Check if user is vendor owner (via company ownership)
+        is_owner = vendor.company and vendor.company.owner_user_id == user.id
 
         if is_owner:
             vendor_role = "Owner"
@@ -121,12 +121,15 @@ def vendor_login(
                 )
     else:
         # No vendor context - find which vendor this user belongs to
-        # Check owned vendors first
-        if user.owned_vendors:
-            vendor = user.owned_vendors[0]
-            vendor_role = "Owner"
-        # Check vendor memberships
-        elif user.vendor_memberships:
+        # Check owned vendors first (via company ownership)
+        for company in user.owned_companies:
+            if company.vendors:
+                vendor = company.vendors[0]
+                vendor_role = "Owner"
+                break
+
+        # Check vendor memberships if no owned vendor found
+        if not vendor and user.vendor_memberships:
             active_membership = next(
                 (vm for vm in user.vendor_memberships if vm.is_active), None
             )