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

contracts/openapi.yaml

@@ -23,7 +23,7 @@ info:
       `protocol_unsupported`, `rate_limited`, `internal`, `unavailable`; unauthenticated ones `rate_limited`,
       `internal`, `unavailable`. `not_implemented` (501) exists only during milestones M2a to M4.
 servers:
-  - url: https://{host}:8443
+  - url: https://{host}
     variables:
       host:
         default: kvitt.example.invalid