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>

models/database/vendor.py

@@ -63,6 +63,17 @@ class Vendor(Base, TimestampMixin):
         Boolean, default=False
     )  # Boolean to indicate if the vendor brand is verified
 
+    # ========================================================================
+    # Contact Information (nullable = inherit from company)
+    # ========================================================================
+    # These fields allow vendor-specific branding/identity.
+    # If null, the value is inherited from the parent company.
+    contact_email = Column(String(255), nullable=True)  # Override company contact email
+    contact_phone = Column(String(50), nullable=True)  # Override company contact phone
+    website = Column(String(255), nullable=True)  # Override company website
+    business_address = Column(Text, nullable=True)  # Override company business address
+    tax_number = Column(String(100), nullable=True)  # Override company tax number
+
     # ========================================================================
     # Relationships
     # ========================================================================
@@ -202,6 +213,66 @@ class Vendor(Base, TimestampMixin):
                 domains.append(domain.domain)  # Add other active custom domains
         return domains
 
+    # ========================================================================
+    # Contact Resolution Helper Properties
+    # ========================================================================
+    # These properties return the effective value (vendor override or company fallback)
+
+    @property
+    def effective_contact_email(self) -> str | None:
+        """Get contact email (vendor override or company fallback)."""
+        if self.contact_email is not None:
+            return self.contact_email
+        return self.company.contact_email if self.company else None
+
+    @property
+    def effective_contact_phone(self) -> str | None:
+        """Get contact phone (vendor override or company fallback)."""
+        if self.contact_phone is not None:
+            return self.contact_phone
+        return self.company.contact_phone if self.company else None
+
+    @property
+    def effective_website(self) -> str | None:
+        """Get website (vendor override or company fallback)."""
+        if self.website is not None:
+            return self.website
+        return self.company.website if self.company else None
+
+    @property
+    def effective_business_address(self) -> str | None:
+        """Get business address (vendor override or company fallback)."""
+        if self.business_address is not None:
+            return self.business_address
+        return self.company.business_address if self.company else None
+
+    @property
+    def effective_tax_number(self) -> str | None:
+        """Get tax number (vendor override or company fallback)."""
+        if self.tax_number is not None:
+            return self.tax_number
+        return self.company.tax_number if self.company else None
+
+    def get_contact_info_with_inheritance(self) -> dict:
+        """
+        Get all contact info with inheritance flags.
+
+        Returns dict with resolved values and flags indicating if inherited from company.
+        """
+        company = self.company
+        return {
+            "contact_email": self.effective_contact_email,
+            "contact_email_inherited": self.contact_email is None and company is not None,
+            "contact_phone": self.effective_contact_phone,
+            "contact_phone_inherited": self.contact_phone is None and company is not None,
+            "website": self.effective_website,
+            "website_inherited": self.website is None and company is not None,
+            "business_address": self.effective_business_address,
+            "business_address_inherited": self.business_address is None and company is not None,
+            "tax_number": self.effective_tax_number,
+            "tax_number_inherited": self.tax_number is None and company is not None,
+        }
+
 
 class VendorUserType(str, enum.Enum):
     """Types of vendor users."""