Server and contracts: public URL on port 443 (D-124)

Kvitt is served on the default HTTPS port through Nginx Proxy Manager, so the
OpenAPI server is https://{host} (contracts/openapi.yaml and openapi.rs
PUBLIC_SERVER_URL), examples E-7 public_url and E-12 link are
https://kvitt.example.invalid without a port, tools/contracts-check.py expects
the portless header, and the server's tests, config message and README name
no port.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

server/crates/kvitt-server/tests/skeleton/support.rs

@@ -49,7 +49,7 @@ impl TestApp {
         let dir = TempDir::new();
         let config = Config::from_lookup(|k| match k {
             "KVITT_DATA_DIR" => Some(dir.0.to_string_lossy().into_owned()),
-            "KVITT_PUBLIC_URL" => Some("https://kvitt.example.invalid:8443".to_owned()),
+            "KVITT_PUBLIC_URL" => Some("https://kvitt.example.invalid".to_owned()),
             _ => None,
         })
         .unwrap();