style: apply black and isort formatting across entire codebase

- Standardize quote style (single to double quotes)
- Reorder and group imports alphabetically
- Fix line breaks and indentation for consistency
- Apply PEP 8 formatting standards

Also updated Makefile to exclude both venv and .venv from code quality checks.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

models/database/cart.py

@@ -1,7 +1,9 @@
 # models/database/cart.py
 """Cart item database model."""
 from datetime import datetime
-from sqlalchemy import Column, Float, ForeignKey, Index, Integer, String, UniqueConstraint
+
+from sqlalchemy import (Column, Float, ForeignKey, Index, Integer, String,
+                        UniqueConstraint)
 from sqlalchemy.orm import relationship
 
 from app.core.database import Base
@@ -15,6 +17,7 @@ class CartItem(Base, TimestampMixin):
     Stores cart items per session, vendor, and product.
     Sessions are identified by a session_id string (from browser cookies).
     """
+
     __tablename__ = "cart_items"
 
     id = Column(Integer, primary_key=True, index=True)