feat(vendor): add contact info inheritance from company

Vendors can now override company contact information for specific branding.
Fields are nullable - if null, value is inherited from parent company.

Database changes:
- Add vendor.contact_email, contact_phone, website, business_address, tax_number
- All nullable (null = inherit from company)
- Alembic migration: 28d44d503cac

Model changes:
- Add effective_* properties for resolved values
- Add get_contact_info_with_inheritance() helper

Schema changes:
- VendorCreate: Optional contact fields for override at creation
- VendorUpdate: Contact fields + reset_contact_to_company flag
- VendorDetailResponse: Resolved values + *_inherited flags

API changes:
- GET/PUT vendor endpoints return resolved contact info
- PUT accepts contact overrides (empty string = reset to inherit)
- _build_vendor_detail_response helper for consistent responses

Service changes:
- admin_service.update_vendor handles reset_contact_to_company flag
- Empty strings converted to None for inheritance

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

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

app/services/admin_service.py

@@ -369,6 +369,21 @@ class AdminService:
             # Get update data
             update_data = vendor_update.model_dump(exclude_unset=True)
 
+            # Handle reset_contact_to_company flag
+            if update_data.pop("reset_contact_to_company", False):
+                # Reset all contact fields to None (inherit from company)
+                update_data["contact_email"] = None
+                update_data["contact_phone"] = None
+                update_data["website"] = None
+                update_data["business_address"] = None
+                update_data["tax_number"] = None
+
+            # Convert empty strings to None for contact fields (empty = inherit)
+            contact_fields = ["contact_email", "contact_phone", "website", "business_address", "tax_number"]
+            for field in contact_fields:
+                if field in update_data and update_data[field] == "":
+                    update_data[field] = None
+
             # Check subdomain uniqueness if changing
             if (
                 "subdomain" in update_data