[codex] Fix backend regex unicode case support

[zerox80]

Summary

• enable the unicode-case feature for the backend regex dependency
• fixes the frontend proxy panic when compiling case-insensitive SEO meta tag regexes

Root Cause

The backend builds regex with default features disabled and only std enabled, but frontend_proxy.rs uses (?i) case-insensitive regexes. In the container this panics at request time, poisoning the lazy regex and causing nginx to return 502 because the upstream closes the connection.

Validation

• cargo check in backend/