admin: per-engine request rate limit configurable in the web UI

Adds a "Rate limit (ms)" column to the Settings per-engine overrides
table for engine_request_min_interval_ms (0 = no limit). Backend GET
returns the map, POST saves it via the int-override sanitizer.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014S8VtAvG499SsCbeESRK7V