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/schema/vendor.py

@@ -25,7 +25,8 @@ class VendorCreate(BaseModel):
     """
     Schema for creating a new vendor (storefront/brand) under an existing company.
 
-    Business contact info is inherited from the parent company.
+    Contact info is inherited from the parent company by default.
+    Optionally, provide contact fields to override from the start.
     """
 
     # Parent company
@@ -51,6 +52,13 @@ class VendorCreate(BaseModel):
     letzshop_csv_url_en: str | None = Field(None, description="English CSV URL")
     letzshop_csv_url_de: str | None = Field(None, description="German CSV URL")
 
+    # Contact Info (optional - if not provided, inherited from company)
+    contact_email: str | None = Field(None, description="Override company contact email")
+    contact_phone: str | None = Field(None, description="Override company contact phone")
+    website: str | None = Field(None, description="Override company website")
+    business_address: str | None = Field(None, description="Override company business address")
+    tax_number: str | None = Field(None, description="Override company tax number")
+
     @field_validator("subdomain")
     @classmethod
     def validate_subdomain(cls, v):
@@ -72,8 +80,8 @@ class VendorUpdate(BaseModel):
     """
     Schema for updating vendor information (Admin only).
 
-    Note: Business contact info (contact_email, etc.) is at the Company level.
-    Use company update endpoints to modify those fields.
+    Contact fields can be overridden at the vendor level.
+    Set to null/empty to reset to company default (inherit).
     """
 
     # Basic Information
@@ -90,6 +98,18 @@ class VendorUpdate(BaseModel):
     is_active: bool | None = None
     is_verified: bool | None = None
 
+    # Contact Info (set value to override, set to empty string to reset to inherit)
+    contact_email: str | None = Field(None, description="Override company contact email")
+    contact_phone: str | None = Field(None, description="Override company contact phone")
+    website: str | None = Field(None, description="Override company website")
+    business_address: str | None = Field(None, description="Override company business address")
+    tax_number: str | None = Field(None, description="Override company tax number")
+
+    # Special flag to reset contact fields to inherit from company
+    reset_contact_to_company: bool | None = Field(
+        None, description="If true, reset all contact fields to inherit from company"
+    )
+
     @field_validator("subdomain")
     @classmethod
     def subdomain_lowercase(cls, v):
@@ -135,16 +155,14 @@ class VendorResponse(BaseModel):
 
 class VendorDetailResponse(VendorResponse):
     """
-    Extended vendor response including company information.
+    Extended vendor response including company information and resolved contact info.
 
-    Includes company details like contact info and owner information.
+    Contact fields show the effective value (vendor override or company default)
+    with flags indicating if the value is inherited from the parent company.
     """
 
     # Company info
     company_name: str = Field(..., description="Name of the parent company")
-    company_contact_email: str = Field(..., description="Company business contact email")
-    company_contact_phone: str | None = Field(None, description="Company phone number")
-    company_website: str | None = Field(None, description="Company website URL")
 
     # Owner info (at company level)
     owner_email: str = Field(
@@ -152,6 +170,25 @@ class VendorDetailResponse(VendorResponse):
     )
     owner_username: str = Field(..., description="Username of the company owner")
 
+    # Resolved contact info (vendor override or company default)
+    contact_email: str | None = Field(None, description="Effective contact email")
+    contact_phone: str | None = Field(None, description="Effective contact phone")
+    website: str | None = Field(None, description="Effective website")
+    business_address: str | None = Field(None, description="Effective business address")
+    tax_number: str | None = Field(None, description="Effective tax number")
+
+    # Inheritance flags (True = value is inherited from company, not overridden)
+    contact_email_inherited: bool = Field(False, description="True if contact_email is from company")
+    contact_phone_inherited: bool = Field(False, description="True if contact_phone is from company")
+    website_inherited: bool = Field(False, description="True if website is from company")
+    business_address_inherited: bool = Field(False, description="True if business_address is from company")
+    tax_number_inherited: bool = Field(False, description="True if tax_number is from company")
+
+    # Original company values (for reference in UI)
+    company_contact_email: str | None = Field(None, description="Company's contact email")
+    company_contact_phone: str | None = Field(None, description="Company's phone number")
+    company_website: str | None = Field(None, description="Company's website URL")
+
 
 class VendorCreateResponse(VendorDetailResponse):
     """