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>

alembic/versions/28d44d503cac_add_contact_fields_to_vendor.py

@@ -0,0 +1,37 @@
+"""add contact fields to vendor
+
+Revision ID: 28d44d503cac
+Revises: 9f3a25ea4991
+Create Date: 2025-12-03 22:26:02.161087
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '28d44d503cac'
+down_revision: Union[str, None] = '9f3a25ea4991'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add nullable contact fields to vendor table
+    # These allow vendor-specific branding/identity, overriding company defaults
+    op.add_column('vendors', sa.Column('contact_email', sa.String(255), nullable=True))
+    op.add_column('vendors', sa.Column('contact_phone', sa.String(50), nullable=True))
+    op.add_column('vendors', sa.Column('website', sa.String(255), nullable=True))
+    op.add_column('vendors', sa.Column('business_address', sa.Text(), nullable=True))
+    op.add_column('vendors', sa.Column('tax_number', sa.String(100), nullable=True))
+
+
+def downgrade() -> None:
+    # Remove contact fields from vendor table
+    op.drop_column('vendors', 'tax_number')
+    op.drop_column('vendors', 'business_address')
+    op.drop_column('vendors', 'website')
+    op.drop_column('vendors', 'contact_phone')
+    op.drop_column('vendors', 'contact_email')