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

alembic/versions/5818330181a5_make_vendor_owner_user_id_nullable_for_.py

@@ -0,0 +1,48 @@
+"""make_vendor_owner_user_id_nullable_for_company_ownership
+
+Revision ID: 5818330181a5
+Revises: d0325d7c0f25
+Create Date: 2025-12-01 20:30:06.158027
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '5818330181a5'
+down_revision: Union[str, None] = 'd0325d7c0f25'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """
+    Make vendor.owner_user_id nullable to support company-level ownership.
+
+    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.
+    """
+    # Use batch operations for SQLite compatibility
+    with op.batch_alter_table('vendors', schema=None) as batch_op:
+        batch_op.alter_column('owner_user_id',
+                              existing_type=sa.INTEGER(),
+                              nullable=True)
+
+
+def downgrade() -> None:
+    """
+    Revert vendor.owner_user_id to non-nullable.
+
+    WARNING: This will fail if there are vendors without owner_user_id!
+    """
+    # Use batch operations for SQLite compatibility
+    with op.batch_alter_table('vendors', schema=None) as batch_op:
+        batch_op.alter_column('owner_user_id',
+                              existing_type=sa.INTEGER(),
+                              nullable=False)