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/schema/company.py

@@ -0,0 +1,155 @@
+# models/schema/company.py
+"""
+Pydantic schemas for Company model.
+
+These schemas are used for API request/response validation and serialization.
+"""
+
+from pydantic import BaseModel, ConfigDict, EmailStr, Field, field_validator
+
+
+class CompanyBase(BaseModel):
+    """Base schema for company with common fields."""
+
+    name: str = Field(..., min_length=2, max_length=200, description="Company name")
+    description: str | None = Field(None, description="Company description")
+    contact_email: EmailStr = Field(..., description="Business contact email")
+    contact_phone: str | None = Field(None, description="Business phone number")
+    website: str | None = Field(None, description="Company website URL")
+    business_address: str | None = Field(None, description="Physical business address")
+    tax_number: str | None = Field(None, description="Tax/VAT registration number")
+
+    @field_validator("contact_email")
+    @classmethod
+    def normalize_email(cls, v):
+        """Normalize email to lowercase."""
+        return v.lower() if v else v
+
+
+class CompanyCreate(CompanyBase):
+    """
+    Schema for creating a new company.
+
+    Requires owner_email to create the associated owner user account.
+    """
+
+    owner_email: EmailStr = Field(
+        ..., description="Email for the company owner account"
+    )
+
+    @field_validator("owner_email")
+    @classmethod
+    def normalize_owner_email(cls, v):
+        """Normalize owner email to lowercase."""
+        return v.lower() if v else v
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class CompanyUpdate(BaseModel):
+    """
+    Schema for updating company information.
+
+    All fields are optional to support partial updates.
+    """
+
+    name: str | None = Field(None, min_length=2, max_length=200)
+    description: str | None = None
+    contact_email: EmailStr | None = None
+    contact_phone: str | None = None
+    website: str | None = None
+    business_address: str | None = None
+    tax_number: str | None = None
+
+    # Status (Admin only)
+    is_active: bool | None = None
+    is_verified: bool | None = None
+
+    @field_validator("contact_email")
+    @classmethod
+    def normalize_email(cls, v):
+        """Normalize email to lowercase."""
+        return v.lower() if v else v
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class CompanyResponse(BaseModel):
+    """Standard schema for company response data."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    name: str
+    description: str | None
+
+    # Owner information
+    owner_user_id: int
+
+    # Contact Information
+    contact_email: str
+    contact_phone: str | None
+    website: str | None
+
+    # Business Information
+    business_address: str | None
+    tax_number: str | None
+
+    # Status Flags
+    is_active: bool
+    is_verified: bool
+
+    # Timestamps
+    created_at: str
+    updated_at: str
+
+
+class CompanyDetailResponse(CompanyResponse):
+    """
+    Detailed company response including vendor count.
+
+    Used for company detail pages and admin views.
+    """
+
+    vendor_count: int = Field(0, description="Number of vendors under this company")
+    active_vendor_count: int = Field(
+        0, description="Number of active vendors under this company"
+    )
+
+
+class CompanyListResponse(BaseModel):
+    """Schema for paginated company list."""
+
+    companies: list[CompanyResponse]
+    total: int
+    skip: int
+    limit: int
+
+
+class CompanyCreateResponse(BaseModel):
+    """
+    Response after creating a company with owner account.
+
+    Includes temporary password for the owner (shown only once).
+    """
+
+    company: CompanyResponse
+    owner_user_id: int
+    owner_username: str
+    owner_email: str
+    temporary_password: str = Field(
+        ..., description="Temporary password for owner (SHOWN ONLY ONCE)"
+    )
+    login_url: str | None = Field(None, description="URL for company owner to login")
+
+
+class CompanySummary(BaseModel):
+    """Lightweight company summary for dropdowns and quick references."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    name: str
+    is_active: bool
+    is_verified: bool
+    vendor_count: int = 0