Added `LDR_DISABLE_RATE_LIMITING` as the canonical name for disabling HTTP rate limiting, matching the `LDR_` env-var convention. The legacy `DISABLE_RATE_LIMITING` still works but emits a deprecation warning. Note: this is distinct from `LDR_RATE_LIMITING_ENABLED`, which controls the adaptive search-engine rate limiter — a different subsystem (#3905).
