feat(server): add TLS certificate support

[calvarado2004]

Summary

• add optional, vLLM-compatible --ssl-certfile and --ssl-keyfile flags to ft serve
• require the certificate and private key as a pair, then pass them to Uvicorn for direct HTTPS serving
• preserve plain HTTP as the default and reject TLS in attached shell mode, whose loopback client currently assumes HTTP
• document HTTPS usage and cover parsing, validation, Uvicorn forwarding, and the unchanged HTTP default

Motivation

FreeToken currently calls Uvicorn without exposing its TLS settings, so deployments must add a separate reverse proxy even when the inference server should terminate TLS directly. This adds the minimal native path while leaving existing deployments unchanged.

Testing

• python -m pytest -q tests/server — 540 passed
• python -m py_compile python/freetoken/server/args.py python/freetoken/server/api_server.py tests/server/test_tls_args.py

I also checked open and closed FreeToken PRs for TLS/SSL/certificate support and inspected every open PR touching the server argument, launch, or API-server paths; no overlapping implementation was found.