fix: protect critical re-export imports from linter removal

Problem:
- Ruff removed 'from app.core.database import Base' from models/database/base.py
- Import appeared "unused" (F401) but was actually a critical re-export
- Caused ImportError: cannot import name 'Base' at runtime
- Re-export pattern: import in one file to export from package

Solution:
1. Added F401 ignore for models/database/base.py in pyproject.toml
2. Created scripts/verify_critical_imports.py verification script
3. Integrated verification into make check and CI pipeline
4. Updated documentation with explanation

New Verification Script:
- Checks all critical re-export imports exist
- Detects import variations (parentheses, 'as' clauses)
- Handles SQLAlchemy declarative_base alternatives
- Runs as part of make check automatically

Protected Files:
- models/database/base.py - Re-exports Base for all models
- models/__init__.py - Exports Base for Alembic
- models/database/__init__.py - Exports Base from package
- All __init__.py files (already protected)

Makefile Changes:
- make verify-imports - Run import verification
- make check - Now includes verify-imports
- make ci - Includes verify-imports in pipeline

Documentation Updated:
- Code quality guide explains re-export protection
- Pre-commit workflow includes verification
- Examples of why re-exports matter

This prevents future issues where linters remove seemingly
"unused" imports that are actually critical for application structure.

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

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

models/database/cart.py

@@ -1,7 +1,6 @@
 # models/database/cart.py
 """Cart item database model."""
 
-
 from sqlalchemy import (
     Column,
     Float,

models/database/customer.py

@@ -1,4 +1,3 @@
-
 from sqlalchemy import (
     JSON,
     Boolean,

models/database/marketplace_import_job.py

@@ -1,4 +1,3 @@
-
 from sqlalchemy import Column, DateTime, ForeignKey, Index, Integer, String, Text
 from sqlalchemy.orm import relationship
 

models/database/marketplace_product.py

@@ -1,4 +1,3 @@
-
 from sqlalchemy import (
     Column,
     Index,

models/database/vendor_domain.py

@@ -3,7 +3,6 @@
 Vendor Domain Model - Maps custom domains to vendors
 """
 
-
 from sqlalchemy import (
     Boolean,
     Column,