- Add _is_trusted_proxy() to check client IP against trusted CIDRs
- Only use X-Forwarded-For if connection came from trusted proxy
- Add TRUSTED_PROXY_CIDRS config (default: 127.0.0.1/32, ::1/128)
- Add parse_comma_separated_lists for env var parsing
- Add 3 unit tests: trusted CIDR check, untrusted ignores forwarded, trusted uses forwarded