Enhance ASGI compatibility, integrate async operations, and update dependencies for improved performance

[jocxfin]

Dockerfile:

• Modified Gunicorn command to support Uvicorn workers: -k "uvicorn.workers.UvicornWorker" for ASGI compatibility.

app.py:

• Introduced asyncio and httpx for async HTTP requests.
• Converted functions to async (check_password_pwned, generate_passphrase, route handlers) for non-blocking operations.
• Updated special_characters to include a wider range of symbols.
• Simplified word list reading with .readlines().
• Added Cache setup for response caching.

requirements.txt:

• Added httpx, flask-caching, uvicorn[standard]>=0.14.0.
• Updated Flask to support async: Flask[async]>=3.0.1.
• Specified gunicorn>=21.2.0 for compatibility.