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

app/exceptions/company.py

@@ -0,0 +1,128 @@
+# app/exceptions/company.py
+"""
+Company management specific exceptions.
+"""
+
+from typing import Any
+
+from .base import (
+    AuthorizationException,
+    BusinessLogicException,
+    ConflictException,
+    ResourceNotFoundException,
+    ValidationException,
+)
+
+
+class CompanyNotFoundException(ResourceNotFoundException):
+    """Raised when a company is not found."""
+
+    def __init__(self, company_identifier: str | int, identifier_type: str = "id"):
+        if identifier_type.lower() == "id":
+            message = f"Company with ID '{company_identifier}' not found"
+        else:
+            message = f"Company with name '{company_identifier}' not found"
+
+        super().__init__(
+            resource_type="Company",
+            identifier=str(company_identifier),
+            message=message,
+            error_code="COMPANY_NOT_FOUND",
+        )
+
+
+class CompanyAlreadyExistsException(ConflictException):
+    """Raised when trying to create a company that already exists."""
+
+    def __init__(self, company_name: str):
+        super().__init__(
+            message=f"Company with name '{company_name}' already exists",
+            error_code="COMPANY_ALREADY_EXISTS",
+            details={"company_name": company_name},
+        )
+
+
+class CompanyNotActiveException(BusinessLogicException):
+    """Raised when trying to perform operations on inactive company."""
+
+    def __init__(self, company_id: int):
+        super().__init__(
+            message=f"Company with ID '{company_id}' is not active",
+            error_code="COMPANY_NOT_ACTIVE",
+            details={"company_id": company_id},
+        )
+
+
+class CompanyNotVerifiedException(BusinessLogicException):
+    """Raised when trying to perform operations requiring verified company."""
+
+    def __init__(self, company_id: int):
+        super().__init__(
+            message=f"Company with ID '{company_id}' is not verified",
+            error_code="COMPANY_NOT_VERIFIED",
+            details={"company_id": company_id},
+        )
+
+
+class UnauthorizedCompanyAccessException(AuthorizationException):
+    """Raised when user tries to access company they don't own."""
+
+    def __init__(self, company_id: int, user_id: int | None = None):
+        details = {"company_id": company_id}
+        if user_id:
+            details["user_id"] = user_id
+
+        super().__init__(
+            message=f"Unauthorized access to company with ID '{company_id}'",
+            error_code="UNAUTHORIZED_COMPANY_ACCESS",
+            details=details,
+        )
+
+
+class InvalidCompanyDataException(ValidationException):
+    """Raised when company data is invalid or incomplete."""
+
+    def __init__(
+        self,
+        message: str = "Invalid company data",
+        field: str | None = None,
+        details: dict[str, Any] | None = None,
+    ):
+        super().__init__(
+            message=message,
+            field=field,
+            details=details,
+        )
+        self.error_code = "INVALID_COMPANY_DATA"
+
+
+class CompanyValidationException(ValidationException):
+    """Raised when company validation fails."""
+
+    def __init__(
+        self,
+        message: str = "Company validation failed",
+        field: str | None = None,
+        validation_errors: dict[str, str] | None = None,
+    ):
+        details = {}
+        if validation_errors:
+            details["validation_errors"] = validation_errors
+
+        super().__init__(
+            message=message,
+            field=field,
+            details=details,
+        )
+        self.error_code = "COMPANY_VALIDATION_FAILED"
+
+
+class CompanyHasVendorsException(BusinessLogicException):
+    """Raised when trying to delete a company that still has active vendors."""
+
+    def __init__(self, company_id: int, vendor_count: int):
+        super().__init__(
+            message=f"Cannot delete company with ID '{company_id}' because it has {vendor_count} associated vendor(s)",
+            error_code="COMPANY_HAS_VENDORS",
+            details={"company_id": company_id, "vendor_count": vendor_count},
+        )