get_issue_report mapped every error from Client::issue_report to 503
(StatusUnavailable) unconditionally. issue_report calls
issue_list_issues(org, repo, ...) first, and a Forgejo 404 there — a
typo'd or deleted repo — was flattened into the same 503 a genuine
forge outage produces, which tells a client to retry a request that
will never succeed.
Downcast the anyhow error back to forgejo_api::ForgejoError (same
pattern main.rs's wanted_error_status uses for
swarm_queue_client::Error) and check it structurally against the
ApiErrorKind::NotFound / UnexpectedStatusCode(404) shapes forgejo's
generated client produces for a 404, rather than string-matching the
rendered message. Only that case answers 404, naming the org/repo;
every other forge failure still answers 503. Updates the route's
OpenAPI response list to document the 404.
Closes#4701
