[markusbrand]

This PR upgrades the mmdGenerator codebase to version 2.0.0.

Key improvements:

• Backend Performance: Refactored the database service to use a long-lived SQLite connection managed by a FastAPI lifespan handler. Added robust error handling.
• Frontend Quality: Shifted diagram exports (PNG/PDF) to the backend for better consistency. Optimized React components with memoization.
• Full QA Suite: Added backend unit tests for services/API and frontend E2E tests using Playwright.
• Release Ready: Bumped versions and tagged v2.0.0.

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PR created automatically by Jules for task 6834955936093424300 started by @markusbrand

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