feat: implement company-based ownership architecture

- Add database migration to make vendor.owner_user_id nullable
- Update Vendor model to support company-based ownership (DEPRECATED vendor.owner_user_id)
- Implement company_service with singleton pattern (consistent with vendor_service)
- Create Company model with proper relationships to vendors and users
- Add company exception classes for proper error handling
- Refactor companies API to use singleton service pattern

Architecture Change:
- OLD: Each vendor has its own owner (vendor.owner_user_id)
- NEW: Vendors belong to a company, company has one owner (company.owner_user_id)
- This allows one company owner to manage multiple vendor brands

Technical Details:
- Company service uses singleton pattern (not factory)
- Company service accepts db: Session as parameter (follows SVC-003)
- Uses AuthManager for password hashing (consistent with admin_service)
- Added _generate_temp_password() helper method

models/database/vendor.py

@@ -35,48 +35,47 @@ class Vendor(Base, TimestampMixin):
     id = Column(
         Integer, primary_key=True, index=True
     )  # Primary key and indexed column for vendor ID
+
+    # Company relationship
+    company_id = Column(
+        Integer, ForeignKey("companies.id"), nullable=False, index=True
+    )  # Foreign key to the parent company
+
     vendor_code = Column(
         String, unique=True, index=True, nullable=False
     )  # Unique, indexed, non-nullable vendor code column
     subdomain = Column(
         String(100), unique=True, nullable=False, index=True
     )  # Unique, non-nullable subdomain column with indexing
-    name = Column(String, nullable=False)  # Non-nullable name column for the vendor
+    name = Column(String, nullable=False)  # Non-nullable name column for the vendor (brand name)
     description = Column(Text)  # Optional text description column for the vendor
+
     owner_user_id = Column(
-        Integer, ForeignKey("users.id"), nullable=False
-    )  # Foreign key to user ID of the vendor's owner
+        Integer, ForeignKey("users.id"), nullable=True
+    )  # Foreign key to user ID of the vendor's owner (DEPRECATED - use company.owner_user_id instead)
 
-    # Contact information
-    contact_email = Column(String)  # Optional email column for contact information
-    contact_phone = Column(String)  # Optional phone column for contact information
-    website = Column(String)  # Optional website column for contact information
-
-    # Letzshop URLs - multi-language support
+    # Letzshop URLs - multi-language support (brand-specific marketplace feeds)
     letzshop_csv_url_fr = Column(String)  # URL for French CSV in Letzshop
     letzshop_csv_url_en = Column(String)  # URL for English CSV in Letzshop
     letzshop_csv_url_de = Column(String)  # URL for German CSV in Letzshop
 
-    # Business information
-    business_address = Column(
-        Text
-    )  # Optional text address column for business information
-    tax_number = Column(String)  # Optional tax number column for business information
-
-    # Status
+    # Status (vendor-specific, can differ from company status)
     is_active = Column(
         Boolean, default=True
-    )  # Boolean to indicate if the vendor is active
+    )  # Boolean to indicate if the vendor brand is active
     is_verified = Column(
         Boolean, default=False
-    )  # Boolean to indicate if the vendor is verified
+    )  # Boolean to indicate if the vendor brand is verified
 
     # ========================================================================
     # Relationships
     # ========================================================================
+    company = relationship(
+        "Company", back_populates="vendors"
+    )  # Relationship with Company model for the parent company
     owner = relationship(
         "User", back_populates="owned_vendors"
-    )  # Relationship with User model for the vendor's owner
+    )  # Relationship with User model for the vendor's owner (legacy)
     vendor_users = relationship(
         "VendorUser", back_populates="vendor"
     )  # Relationship with VendorUser model for users in this vendor