fix: sync pyproject.toml dependencies with requirements.txt and add CodeQL SAST workflow

[namann5]

Summary

The pyproject.toml declares runtime dependencies that are inconsistent with backend/requirements.txt. Three critical packages — cryptography, httpx, and openai — are present in requirements.txt but missing from pyproject.toml.

This means any installation via pip install . or future PyPI publication silently omits these packages, causing runtime failures in core features:

Missing Package	Broken Feature	Impact
cryptography>=42.0.0	VaultCrypto AES-256-GCM encryption (backend/secuscan/vault.py)	Credential vault raises ImportError; all stored secrets become inaccessible
httpx>=0.28.1	Webhook notification delivery (backend/secuscan/notification_service.py)	All webhook/Slack notifications silently fail
openai>=1.0.0	AI Executive Summary (backend/secuscan/ai_summary.py)	AI summary feature raises ImportError at call time

Steps to Reproduce

1. pip install . (or create a fresh venv and install from pyproject.toml)
2. Start SecuScan
3. Attempt to use the credential vault, webhook notifications, or AI summary
4. Observe ModuleNotFoundError / ImportError at runtime

Root Cause

pyproject.toml at line 11-22 lists 11 dependencies. backend/requirements.txt at line 2, 13-14 lists 3 additional packages (cryptography, httpx, openai) that are imported and used at runtime but never declared in the project metadata.

Proposed Fix

PR adds the missing dependencies to pyproject.toml:

dependencies = [
    ...
    "cryptography>=42.0.0",
    "httpx>=0.28.1",
    "openai>=1.0.0",
]

Additionally adds a CodeQL security analysis workflow for CI SAST scanning and a .python-version file for consistent local development.

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