refactor: Complete backend clean architecture (Phase 4)

BREAKING CHANGE: app.py refactored from 1832 to ~200 lines

- Replaced monolithic app.py with clean architecture version
- Added sys.path setup for dual execution modes
- Early load_dotenv before imports
- All routers now mounted via include_router()
- Removed inline route definitions
- Kept only FastAPI init, middleware, lifecycle hooks

Verified working:
- /health returns {"status": "healthy"}
- /api/templates returns templates array
- /docs Swagger UI shows all routers

Note: App must now run as 'uvicorn backend.app:app' from project root

Author: cliff-de-tech