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/examples/E-12-create-invite/response.json

@@ -3,7 +3,7 @@
   "kind": "partner",
   "target_user_id": null,
   "code": "4N8RVT2KW6ZC",
-  "link": "https://kvitt.example.invalid:8443/i/4N8RVT2KW6ZC?from=Jacob",
+  "link": "https://kvitt.example.invalid/i/4N8RVT2KW6ZC?from=Jacob",
   "created_at": "2026-10-17T06:30:02.004Z",
   "expires_at": "2026-10-20T06:30:02.004Z"
 }