Add dependency vulnerability scanning with pip-audit or uv audit

[qartik]

Introduce dependency vulnerability scanning using pip-audit or uv audit to detect known issues in Python dependencies.

Scope:

• Add pip-audit or uv audit to CI (GitHub Actions)
• Fail builds on known vulnerabilities (configurable threshold)
• Optionally generate a report artifact

Acceptance criteria:

• CI runs pip-audit or uv audit on each PR
• Fails when vulnerabilities are detected
• Documentation added for local usage (both options or chosen standard)

References:
https://github.com/pypa/pip-audit
https://github.com/astral-sh/uv