Add `http_curl.h` providing `curl_post` function using libcurl, update `openai.h` to call `curl_post` instead of `https_post` in both `openai_system` and `openai_chat`, add missing `#ifndef`/`#endif` include guard to `auth.h`, increase HTTP buffer size from 4096 to 1MB, add explicit `(char *)` casts to `malloc`/`realloc` calls in `http.h`, and link `-lcurl` in both `build` and `build_free` Makefile targets.
