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/company.py

@@ -0,0 +1,106 @@
+# models/database/company.py
+"""
+Company model representing the business entity that owns one or more vendor brands.
+
+A Company represents the legal/business entity with contact information,
+while Vendors represent the individual brands/storefronts operated by that company.
+"""
+
+from sqlalchemy import Boolean, Column, ForeignKey, Integer, String, Text
+from sqlalchemy.orm import relationship
+
+from app.core.database import Base
+from models.database.base import TimestampMixin
+
+
+class Company(Base, TimestampMixin):
+    """
+    Represents a company (business entity) in the system.
+
+    A company owns one or more vendor brands. All business/contact information
+    is stored at the company level to avoid duplication.
+    """
+
+    __tablename__ = "companies"
+
+    # ========================================================================
+    # Basic Information
+    # ========================================================================
+    id = Column(Integer, primary_key=True, index=True)
+    """Unique identifier for the company."""
+
+    name = Column(String, nullable=False, index=True)
+    """Company legal/business name."""
+
+    description = Column(Text)
+    """Optional description of the company."""
+
+    # ========================================================================
+    # Ownership
+    # ========================================================================
+    owner_user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    """Foreign key to the user who owns this company."""
+
+    # ========================================================================
+    # Contact Information
+    # ========================================================================
+    contact_email = Column(String, nullable=False)
+    """Primary business contact email."""
+
+    contact_phone = Column(String)
+    """Business phone number."""
+
+    website = Column(String)
+    """Company website URL."""
+
+    # ========================================================================
+    # Business Details
+    # ========================================================================
+    business_address = Column(Text)
+    """Physical business address."""
+
+    tax_number = Column(String)
+    """Tax/VAT registration number."""
+
+    # ========================================================================
+    # Status Flags
+    # ========================================================================
+    is_active = Column(Boolean, default=True, nullable=False)
+    """Whether the company is active. Affects all associated vendors."""
+
+    is_verified = Column(Boolean, default=False, nullable=False)
+    """Whether the company has been verified by platform admins."""
+
+    # ========================================================================
+    # Relationships
+    # ========================================================================
+    owner = relationship("User", back_populates="owned_companies")
+    """The user who owns this company."""
+
+    vendors = relationship(
+        "Vendor",
+        back_populates="company",
+        cascade="all, delete-orphan",
+        order_by="Vendor.name",
+    )
+    """All vendor brands operated by this company."""
+
+    def __repr__(self):
+        """String representation of the Company object."""
+        return f"<Company(id={self.id}, name='{self.name}', vendors={len(self.vendors) if self.vendors else 0})>"
+
+    # ========================================================================
+    # Helper Properties
+    # ========================================================================
+
+    @property
+    def vendor_count(self) -> int:
+        """Get the number of vendors belonging to this company."""
+        return len(self.vendors) if self.vendors else 0
+
+    @property
+    def active_vendor_count(self) -> int:
+        """Get the number of active vendors belonging to this company."""
+        if not self.vendors:
+            return 0
+        return sum(1 for v in self.vendors if v.is_active)