fix: regenerate TLS cert when services change (hot-reload via ArcSwap)

HTTPS proxy certs were generated once at startup. Services added at
runtime via API or LAN discovery got "not secure" in the browser
because their SAN wasn't in the cert. Now the cert is regenerated
on every service add/remove and swapped atomically via ArcSwap.
In-flight connections are unaffected.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -27,3 +27,4 @@ rcgen = { version = "0.13", features = ["pem", "x509-parser"] }
 time = "0.3"
 rustls = "0.23"
 tokio-rustls = "0.26"
+arc-swap = "1"