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/config.rs

@@ -69,7 +69,7 @@ impl Config {
             AppError::new(
                 crate::error::ErrorCode::Internal,
                 "KVITT_PUBLIC_URL is not set: it must be the Kvitt base URL, \
-                 https://<KVITT_PUBLIC_HOST>:8443 (SPEC §10.4)",
+                 https://<KVITT_PUBLIC_HOST> (SPEC §10.4)",
             )
         })?;
         Ok(Self {
@@ -102,7 +102,7 @@ impl Config {
 mod tests {
     use super::*;
 
-    const PUBLIC_URL: &str = "https://kvitt.example.invalid:8443";
+    const PUBLIC_URL: &str = "https://kvitt.example.invalid";
 
     #[test]
     fn defaults_when_unset_or_empty() {