Fix cryptography CVE-2026-69247

[ichwars]

Summary

• upgrade cryptography to 50.0.0, the first release patched for CVE-2026-69247
• upgrade pyOpenSSL to 26.4.0, the first release supporting cryptography 50.x
• regenerate runtime and development hash lockfiles
• remove the obsolete CVE-2026-69247 pip-audit exception

Local verification

• dependency boundary check
• hash-locked development install
• pip check
• cryptography/pyOpenSSL imports and Fernet round-trip
• pip-audit without a CVE-2026-69247 ignore: no known vulnerabilities
• focused crypto/TLS suite: 134 passed, 70 skipped
• full backend suite: 7,532 passed, 245 skipped
• ruff backend lint
• workflow YAML parse and git diff checks

Local Docker validation was unavailable because the Docker daemon is not running; required CI Docker build, integration tests, and Trivy scan cover that boundary.