feat: Cascade reuse optimization + HTTPS proxy + configurable pool size (#181 by @Fermiz)

- Add HTTPS reverse proxy (HTTP/2 + HTTP/1.1) with CORS for Office plugins
- Add CASCADE_REUSE_BY_CALLER env var for session-pinned cascade reuse
- Add CASCADE_POOL_MAX env var to configure pool size (default 500)
- Add modelKey to pool entries for caller-based fallback lookup
- Add local-setup.md with step-by-step HTTPS setup guide
- Add *.pem to .gitignore

Co-authored-by: Fermi Li <epam.fermi_li@mercedes-benz.com>