feat: add login anomaly alerts

[tungdd2710]

Summary

Implements login anomaly detection and suspicious activity alerts for #124.

• records hashed IP/user-agent fingerprints on successful login
• flags new networks and new devices against recent login history
• creates stored security alerts with read/unread handling
• returns a login security alert payload and shows a destructive warning toast in the frontend
• documents the API in OpenAPI and README
• makes backend tests hermetic with an in-memory Redis double

Validation

• uv run --python 3.11 --with-requirements requirements.txt python -m pytest -q
• uv run --python 3.11 --with-requirements requirements.txt black --check app tests
• uv run --python 3.11 --with-requirements requirements.txt flake8 app tests
• npm test -- SignIn.test.tsx
• npm run lint
• npm run build

Closes #124.