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/src/openapi.rs

@@ -37,7 +37,7 @@ strings. Common rules: SPEC §14.3.1.";
 
 /// The production base URL (SPEC §7, §14.3): the host is the server variable [`HOST_VARIABLE`], because the Kvitt
 /// host name is owner-chosen (SPEC §10.5).
-pub const PUBLIC_SERVER_URL: &str = "https://{host}:8443";
+pub const PUBLIC_SERVER_URL: &str = "https://{host}";
 /// The server variable in [`PUBLIC_SERVER_URL`].
 pub const HOST_VARIABLE: &str = "host";
 /// The default of [`HOST_VARIABLE`]: a reserved name that never resolves (RFC 6761), not a real host.