Production-tested Flask patterns with application factory, Blueprints, and SQLAlchemy, preventing 9 documented errors.

• Covers application factory pattern, extension initialization, blueprint organization, and database models to avoid circular imports and context errors
• Prevents known issues including stream_with_context teardown regressions, async/gevent conflicts, CSRF cache interference, and Flask-Login session protection edge cases
• Includes authentication patterns with Flask-Login and Flask-WTF, API route examples, and testing setup with pytest fixtures
• Requires Python 3.9+; verified against Flask 3.1.2, Flask-SQLAlchemy 3.1.1, Werkzeug 3.1.5, and related extensions as of January 2026