feat: basic rate limits

[theuargb]

Implements #357

Main limitation: redis-only.

• add Redis-backed AI rate limiting with per-role env knobs (user/editor/admin) plus fallback defaults
• enforce limits on chat and temporary chat endpoints; surface localized, user-friendly rate-limit UI

[vercel]

@theuargb is attempting to deploy a commit to the cgoinglove's projects Team on Vercel.

A member of the Team first needs to authorize it.