Adds a separate API-token section under /users/settings (distinct from
the beacon-monitor token list, since API tokens grant full account
access). The plaintext is surfaced exactly once via flash on creation;
only the SHA-256 hash is persisted, so revocation is the only path back
if the user loses it.
Also fixes the openapi.yaml link on /docs/api: the relative path
resolved to /docs/openapi.yaml from a no-trailing-slash URL and 404'd.
