Move X-BanGUI-Request header name/value to backend/app/utils/constants.py as single source of truth. Add GET /api/v1/config/security-headers endpoint. Update csrf middleware, frontend api client, and docs to use shared constants.
- Add SecurityHeadersMiddleware to backend/app/main.py
- Implements Content-Security-Policy: default-src 'self'
- Implements X-Frame-Options: DENY (clickjacking protection)
- Implements X-Content-Type-Options: nosniff (MIME-sniffing protection)
- Implements X-XSS-Protection: 1; mode=block (browser XSS filters)
- Add CSP meta tag to frontend/index.html for defense-in-depth
- Create Docs/Security.md with comprehensive security headers documentation
- Add test suite (backend/tests/test_security_headers_middleware.py) with 5 tests
- Tests verify headers are present on success and error responses
- Tests ensure all four security headers are correctly set
- All existing tests continue to pass
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
